Consonance and dissonance
A dissonant interval or chord is one which sounds unstable.  It may even sound harsh by itself.  The notes seem to clash.  It cannot be the end of the piece of music (if the music is a normal, traditional tonal piece).  It sounds as if it wants to move on to a consonant interval (this is called "resolving" onto a consonant chord).  The notes C and Fsharp together make a dissonant interval.  The chord C - Dsharp - Fsharp is a dissonant interval: it is full of tension, and it sounds as if it wants to "resolve" onto C - E - G.