~겠다 has to meaning. One is expressing will, and the other is expressing guessing. In this context, it means "I guess I don't understand" or "I don't think I understand", or something like that.

On a side not, tense doesn't necessarily tell the same time as the tense is "supposed" to. For example "If I won a million dollars, I would...": it is written in the past tense, but it is used for present time.

No, that is not a future tense but present