What does it mean to overmix? I have a cheddar bay biscuit mix made by Red Lobster. On the instructions, it says not to overmix the grated cheese, water, and biscuit dough, but it says to mix it. What does that mean? How do I know how much mixing is too much?

I did read the other questions that have to do with overmixing. But they mostly concern muffins, and I'm not sure if it is the same with biscuits.
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.