Lemon balm has a mild lemony flavor with a delicate mint overtone.  Steeping some leaves in boiling water for a few minutes gives you a nice tea that some folks claim will lift your mood and sharpen your thinking, and in fact there are  credible claims for its medicinal value.  But let's look at its value in the kitchen.  In any dish in which a lemony flavor would be welcome, lemon balm is a gentle way to add it - fish, for example, or to freshen a green salad.  Try it in place of basil in a pesto.  Use a lot: it's quite mild.