A cool technique used by guitarists where they bend a string with their fretting hand as it is being plucked/picked in order to raise the pitch as the note decays.

Here are some tips on how to do this best:

1. Bring your thumb closer to/on the top of the fretboard and bend the string towards your thumb as it is easier to squeeze the string towards you than pull it away and it works either way.
2. There are different levels of bends. For example bending a note one half step would make it sound like the note one fret above it. A whole step would be 2 frets.

In common notation in tablature it is shown as a small arrow going up from the note you're meant to bend.