Proofing the yeast means you combine it with water and something sweet. The sugar feeds the yeast and the water hydrates it. This revive the yeast, which you can witness by looking at the liquid: it should become frothy and a little foamy on top too. It also simply looks active. If you don't see any signs of activities, then this might mean the yeast is not fresh and happy anymore, which might not make your dough rise very well. In that case, get some more yeast!