"To activate the yeast" means to awaken and make the yeast active or alive. In cooking or baking, it usually refers to adding water or warmth to dried yeast to stimulate its growth and make it ready for use in the recipe. Full definition
If you would prefer to activate the yeast first, warm 1/2 cup of the milk to 110 - 115 degrees F, stir in the yeast and let sit for a few minutes. (redstaryeast.com)
Like, if you don't activate the yeast properly. (ohladycakes.com)