A hidden agenda is like an ulterior motive; you or someone else may do something or act a certain way in supposed benevolence, but the real reason you or they actually did that was also for personal gain; not to just help out.
My dad kindly lent me the money to buy my first car, but it turned out he just did that so he could borrow it from me all the time. This new car had a towbar to use with his friends trailer he borrowed, so he could move his stuff out faster. Hence, he only lent me the money so that he could use the car I bought, for himself. That guy is full of hidden agendas.