It's not a term of art that I am familiar with in the context of a pizza sauce, but I would say that it is a thin sauce, or possibly one with too much water that you can get puddling in the middle of the pizza.
A thin pizza sauce is sometimes intentional, as I discovered some time ago when I was reading about a pizza sauce used by a professional pizza operator on NY style pizzas. Note, for example, the reference to the thin sauce, and the reasons behind it, in the following:
Remember when it comes to sauce and dry spices... less spice is better. Many people have tried to duplicate NY Style pizza sauce.. they even dumpster dive for secrets and there are non. They always over-spice their sauce. Take 2 cups of Full Red pizza sauce and add a cup of water to it (maybe a bit more). This is a thin sauce for the same reason the dough is a moist dough.. the high heat will boil the water out of the sauce.. too thick of a sauce and you will have tomato paste under your toppings and it will be sickeningly pasty on the edges. Add 1/2 tsp of oregano, 1/2 tsp of basil, 1/4 tsp ground black pepper and 1/4 tsp of crushed red pepper, 1/2 tsp salt, maybe 1/2 tsp of sugar if you think it has too much of an acid bite to it and finally 1 tbs of the fresh grated romano cheese. Stir and let set an hour or better yet overnight.
Some operators go in the opposite direction and use thick sauces, especially if they do a lot of pre-saucing of their skins in advance of actual orders. A thick sauce is also often used for take-and-bake pizzas, where, as in the previous example, you don't want the sauce to migrate down into the dough because it sits around too long before being baked.
BTW, a common test to tell whether a pizza sauce is too thin (or "wet") is to put a few tablespoons on a plate and let it set for about 15 minutes. If too much water separates from the solids, that would usually not be considered a quality pizza sauce.