Meaning

• To 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."

• To believe, to think one grasps sufficiently despite potentially incomplete knowledge.

  "I understand that you have a package for me?"

• (obsolete outside circus, acrobatics) To stand underneath, to support.