There's two answers here, the technical one and the nuanced one.
Technically, an aggressive call of any sort means a call that is made to take a high risk/high reward result. For example, in this auction:
2S promises 6-9 points and 3+ spades. A game in spades needs 25ish points to be reasonably safe, so if you make 4s call with 16 points, that would be aggressive - you're knowing that either you're hoping to see 9 points in dummy, or you're going to be in a challenging game that's more likely to go down.
Typically, aggressive bids aren't recommended; some will bid more aggressively when they feel they need higher variance in order to succeed. However, slightly more aggressive bids are appropriate in some cases, such as at IMPs versus MPs - where that game bonus is critical.
Conservative is the inverse of aggressive, and implies you take safer lines that are less likely to be risky, but less rewarding. It's also not recommended - you will fail to bid games you should be in. Conservative players might pass there with 15 points.
The actual opposite of aggressive would be disciplined, meaning making calls as described in your partnership's convention card. Disciplined players would know how to ask questions and find out whether 4S was makeable (perhaps 3S, or perhaps they have other options in their quiver).
The nuanced answer is that typically when someone describes your bidding generally as aggressive, they're suggesting you bid higher than you should, or when you shouldn't at all.
Being an aggressive overcaller is not uncommon in newer players, as it's tempting to want to get in the action, or to see your hand and feel like you have something you want to describe, but shouldn't.
It is also, however, common that in some bridge clubs, especially with older players used to a very different game, that overcalling at what would be considered a modern norm is considered aggressive.
It's hard to say if you're a "too aggressive" overcaller or a "correctly aggressive" overcaller, though, without seeing your play.