It means to "activate" as yeast makes things rise. Usually once you have put the yeast into the rest of the ingredients you let it "breath" at room temperature and sometimes (depending on the bread) the bread dough is put into the fridge.
yeast is mixed with warm water to make it bloom. this is not done in all recipes. most recipes the yeast will activate with other ingrediants, and usually left to rise ,or stand for some time if yeast is used it is NOT put in refrigerator as yeast needs warmth to work
If you are using a rapid rise yeast you do not have to bloom the yeast, just add it with the dry ingredients. Follow the package instructions for water temperatures.
Letting your bread dough proof (rise) in the refrigerator overnight brings lots of flavor to your bread. Let it come to room temperature (& proper doubling) before baking
Yeast mixed with warm water, not more than 115 degrees F, and a small amount of sugar or honey or maple syrup will cause the yeast to bloom. Allow at least five minutes to bloom and then proceed with adding other ingredients.
Sometimes fruit juice or milk are used as the warmed liquid. In that case a sweetener is not necessary.