Abare is a japanese word that means to violently act or to struggle. In fighting games, it is a term for doing risky actions. The most common version of this is attacking with a fast attack while at frame disadvantage aka "mashing".
While this has a negative connotation, and is risky, this doesn't mean that you should never do it. There is a time and place for this, and it relies on you reading your opponent's next move.
When to Do It?
This depends on what your opponent will do
Ultimately it's a lopsided betting game - your opponent has a lot more available options, but the riskier options are more rewarding, both for you and your opponent.