When you are very very busy you have an aggressive schedule. T_T

Without seeing the context, I would guess that it means a busy, high-pressure schedule. A more common collocation in the UK is "a punishing schedule".

In the U.S., in the context of project management within a company, it means a short schedule, a tight schedule, a schedule with barely enough time. It means a schedule that probably can only be met with luck, and by demanding that employees work overtime. The opposite is of "aggressive schedule" is "realistic schedule." "I know it's an aggressive schedule, but in order to get the contract I had to promise the customer we could do it by July." "That's a very aggressive schedule, I don't believe you can meet it."