user7726: In a recent game I swapped off a knight for 3 pawns:
[Event "County Match"]
[Site "Deepest Darkest Southern England"]
[Date "2018.01.20"]
[White "English Player"]
[Black "Ian Bush"]
[Result "0-1"]
[FEN ""]
[StartPly "41"]

1.c4 Nf6 2.Nf3 g6 3.g3 Bg7 4.Bg2 O-O 5.O-O d6 6.Nc3 c5 7.a3 Nc6 8.Rb1 Bf5 
9.d3 Nd7 10.Nd5 Nb6 11.Ne3 Bd7 12.b4 Qc8 13.b5 Nd4 14.Nxd4 Bxd4 15.a4 Bh3 
16.a5 Bxg2 17.Kxg2 Bxe3 18.Bxe3 Nd7 19.Bh6 Re8 20.Qd2 Nf6 21.h3 Qf5 22.e4 
Nxe4 23.dxe4 Qxe4+ 24.Kh2 Qxc4 25.Rfe1 Rad8 26.Re3 f6 27.Rbe1 e5 28.Rd3 
Re6 29.Rc3 Qxb5 30.Qd5 Rde8 31.Rb3 Qc6 32.Qxc6 bxc6 33.Rb7 R6e7 34.a6 Kf7 
35.Be3 Ke6 36.f4 Rxb7 37.axb7 Rb8 38.Rb1 a5 39.fxe5 fxe5 40.Kg2 a4 41.Kf2 
a3 42.Ke2 a2 43.Ra1 Rxb7 44.Rxa2 c4 45.Kd2 d5 46.Kc3 Rb3+ 47.Kd2 d4 48.Bf2
Kd5 49.Ra5+ Ke4 50.Rc5 c3+ 51.Kc1 d3 52.Rxc6 d2+ 53.Kc2 Rb2+ 54.Kd1 Rb1+ 0-1

I've started here as this was the plan after Qf5, and I played this way as I thought it a good way to get winning chances - it was a match and as it all looked fairly even wins were the order of the day. But Stockfish really doesn't like it, putting me down ~1.5 pawns, and I don't really understand why. Can somebody turn the "thoughts" of the metal monster into human terms for me?

user15504: Counting material is only one portion of evaluating a position. Stockfish considers space, piece mobility and activity, king safety, etc.
All of that produces an evaluation of the position, and for this one SF grades it (using LiChess) as about +2.2 (depth 31).
I note that before your knight sac Stockfish thinks you were already down by about 1.2, despite roughly equal material. That bishop on h6 controlling your weakened dark squares, for example, is clearly worth more than your knight.
As for your decision to sac the knight, from a practical point of view it seems a reasonable choice in a bad situation, because if you simply move your queen to e6 or e5, as SF suggests, white has a clear and natural plan of f4 followed by eventually opening up that file, and your clumsy rooks will not be nimble defenders.
SF suggests for white an immediate b6 after your queen takes on c4, and with that you will not get the pawn storm as you did in the game. White will have an open line to invade with his rooks (which one depends on your reaction to b6) and his extra piece will tell.

7726: <p>In a recent game I swapped off a knight for 3 pawns:</p>

<pre><code>[Event "County Match"]
[Site "Deepest Darkest Southern England"]
[Date "2018.01.20"]
[White "English Player"]
[Black "Ian Bush"]
[Result "0-1"]
[FEN ""]
[StartPly "41"]

1.c4 Nf6 2.Nf3 g6 3.g3 Bg7 4.Bg2 O-O 5.O-O d6 6.Nc3 c5 7.a3 Nc6 8.Rb1 Bf5 
9.d3 Nd7 10.Nd5 Nb6 11.Ne3 Bd7 12.b4 Qc8 13.b5 Nd4 14.Nxd4 Bxd4 15.a4 Bh3 
16.a5 Bxg2 17.Kxg2 Bxe3 18.Bxe3 Nd7 19.Bh6 Re8 20.Qd2 Nf6 21.h3 Qf5 22.e4 
Nxe4 23.dxe4 Qxe4+ 24.Kh2 Qxc4 25.Rfe1 Rad8 26.Re3 f6 27.Rbe1 e5 28.Rd3 
Re6 29.Rc3 Qxb5 30.Qd5 Rde8 31.Rb3 Qc6 32.Qxc6 bxc6 33.Rb7 R6e7 34.a6 Kf7 
35.Be3 Ke6 36.f4 Rxb7 37.axb7 Rb8 38.Rb1 a5 39.fxe5 fxe5 40.Kg2 a4 41.Kf2 
a3 42.Ke2 a2 43.Ra1 Rxb7 44.Rxa2 c4 45.Kd2 d5 46.Kc3 Rb3+ 47.Kd2 d4 48.Bf2
Kd5 49.Ra5+ Ke4 50.Rc5 c3+ 51.Kc1 d3 52.Rxc6 d2+ 53.Kc2 Rb2+ 54.Kd1 Rb1+ 0-1
</code></pre>

<p>I've started here as this was the plan after Qf5, and I played this way as I thought it a good way to get winning chances - it was a match and as it all looked fairly even wins were the order of the day. But Stockfish really doesn't like it, putting me down ~1.5 pawns, and I don't really understand why. Can somebody turn the "thoughts" of the metal monster into human terms for me?</p>
 15504: <p>Counting material is only one portion of evaluating a position. Stockfish considers space, piece mobility and activity, king safety, etc.</p>

<p>All of that produces an evaluation of the position, and for this one SF grades it (using LiChess) as about +2.2 (depth 31).</p>

<p>I note that before your knight sac Stockfish thinks you were already down by about 1.2, despite roughly equal material. That bishop on h6 controlling your weakened dark squares, for example, is clearly worth more than your knight.</p>

<p>As for your decision to sac the knight, from a practical point of view it seems a reasonable choice in a bad situation, because if you simply move your queen to e6 or e5, as SF suggests, white has a clear and natural plan of f4 followed by eventually opening up that file, and your clumsy rooks will not be nimble defenders.</p>

<p>SF suggests for white an immediate b6 after your queen takes on c4, and with that you will not get the pawn storm as you did in the game. White will have an open line to invade with his rooks (which one depends on your reaction to b6) and his extra piece will tell.</p>
