Taste is signed by tapping your middle finger on your chin (or on your lips, as some people do). You take an open palm, indent your middle finger slightly, and tap it on your chin or lips.
The same sign is used for favorite (with the finger tapped on the chin).
Taste is a useful concept with older toddlers, because it introduces the concept of choice and making decisions. For snacks, you can give your toddler a couple of different options and ask, "Which taste do you like?" When they respond, you can ask them to append taste to their answer. For example, signing back, "I like the sweet taste," instead of just, "sweet."
As parents, it is our job to introduce our children to a variety of foods that enhance their health, nutrition, and appreciation for diversity. The sign for taste or favorite can be introduced in tandem with new food items.