1.verbTo grasp a concept fully and thoroughly, especially (of words, statements, art, etc.) to be aware of the meaning of and (of people) to be aware of the intent of.
"I'm sorry. I don't understand."
2.verbTo believe, to think one grasps sufficiently despite potentially incomplete knowledge.
"I understand that you have a package for me?"
3.verb(obsolete outside circus, acrobatics) To stand underneath, to support.