For example, I understand the weather if I am able to predict and give an explanation of some of its features.
Or, a psychiatrist understands another person if he knows his anxieties and their causes, and can give him useful advice on how to minimise the anxiety.
I understand a command if I know who gave it, what is expected, and whether the command is legitimate.
Also one can understand a reasoning, an argument, and a language.
See also knowledge, epistemology, and thought.