What does it mean to overmix?
Mixing strengthens the gluten structure in recipes that use wheat flours.  In breads, for example, this is a good thing, as that structure is what allows gasses to be trapped inside, and provides the pleasant chew that we associate with a well-made loaf.  However, when making biscuits (or other baked goods where a softer, more crumbly texture is desired...pancakes also come to mind), it is advised to mix ingredients until just incorporated (or...don't over mix).  This is so that you don't develop the gluten structure in your dough.  So, just mix gently and minimally until the ingredients are well-dispersed.