Having finger dexterity helps in things like playing musical instruments, but it also helps in everyday activities. Having dexterous fingers means having hands that are strong and limber. It also means you have complete control over your fingers. For example, if you bend your pinky finger and your ring finger bends with it, your dexterity can use some work.
If you have a guitar, you can do many useful exercises using the neck and the strings. For example, start at the bottom string. Place your fingers on the string, each one in a different fret. Play the cord, applying pressure in a different fret each time you play it. Then move to the next string and repeat the exercise. Move all the way to the top string and then back down. Increase your speed as you get better at it.
Clasp your hands together in front of your chest. The fingers will lace in the way that is most natural to you. Then switch them. It will feel unnatural. Then switch them back and forth. Do this slowly, and then gradually speed it up.
Learning to split your fingers not only stretches them, but allows them to gain independence from each other. An example of this exercise is to group your pointer and middle finger together as well as your ring and pinky fingers. Separate them so there is a gap between your middle finger and ring finger. Then put the fingers back together and do it again.
An easy stretch that helps dexterity is to clench your hands together, then extend your fingers out as far as you can. Do not strain your fingers; just put light tension on them. Then clench and repeat.