Oh boy, you've asked a great question that leads to one of the more confusing aspects of Japanese verbs and their relationship to particles. To answer you question simply, it is not a direct object in the Japanese language. You cannot actively "understand"; rather it's something that just "happens" (through your hard work of course, but it is a result, not an active action).
To get more in depth, this is the difference between what are called "transitive" and "intransitive" verbs. Transitive verbs, overall, take the particle を and have a direct object, while intransitive verbs, overall, take the particle が. Let me use an easy example:
"To start" has one pair: 始まる（はじまる）(intransitive) and 始める（はじめる） (transitive).
The first more or less means "something starts" while the second means "to start."
The teacher started the class. (Here, "teacher" is the subject and "class" is the direct object.)
The class started. (Here, there is no object--rather, "class" is the subject.)
Let me know if you want a more thorough explanation! This is a great question to be asking.