Bloom: verb. use cold liquid to soften gelatin before adding to a hot liquid, allowing gelatin to dissolve.
'Bloom' is also a term used to describe the stiffness of the gelatin, which can be graded at different levels. If I call for gelatin in a recipe, unless otherwise noted, I'm talking about the unflavored gelatin packets you can buy at the store.
Here's how you do it. Pour the liquid called for in your recipe, room temperature or cold but not warm, into a bowl or ramekin. Sprinkle the gelatin on top of the liquid- your goal is to keep large clumps from forming. You don't need to stir it, just set it aside for a few minutes to bloom.
Once the gelatin has absorbed the liquid, it will be a solid-gel substance and spring back if you touch it with your finger. It may also be emitting a nasty odor- it is an animal-based product, and it's not a good smell. It will go away once added to your recipe, I promise.
Be sure to whisk your gelatin very thoroughly into the hot liquid in your recipe, to avoid clumps. Clumps are gross.