Trills are a great way to condition the fret hand, building finger strength, speed, dexterity and independence.�
You can hear a trill in any style of music, but they have a special place in classical, metal, and blues.
Even if you don't end up using this to decorate your playing, it's very useful to boost your technique.
Let's try an example trill between G and Ab on the first string. Basically, it's just a hammer-on and pull off, repeated for awhile. The short hand way of notating it is just to write the letters "tr" over the note.��
You can trill your way up a scale. I'll start one from F.
You can also do trills with different fingers to give them each a workout. So you can use your pinky, that one is pretty hard. You can use your third finger or your second finger.
You can trill any two notes together, and you can trill a note with a note above it or a note below it.�
The trill seems to really condition the fast-action muscles in the fingers in a way that most exercises don't. So practicing this exercise can really help you build your guitar dexterity.