As mentioned by poster 1, it's TNLNSL.  It's shorthand to indicate that, upon finding the geocache, you Took Nothing from the contents of the geocache container, Left Nothing behind in the geocache container, and Signed the Logbook that was in the geocache.  Put the capitalized letters together, and you get TNLNSL.  Another common one is TFTC (Thanks For The Cache).