The float test is a method bakers use to judge whether their sourdough starter is ready to bake with.
This involves dropping a glob of sourdough starter into a glass of water, to see whether it floats.
If the glob of sourdough starter floats, it is ready to bake sourdough bread.
Once your starter is more than 2 weeks old, it is most likely ready to bake with, which is where float testing would come in to judge its level of preparedness.