Abare is a japanese word that means to violently act or to struggle. In fighting games, it is general a term for doing risky actions. The most common version of this is using a fast attack while at frame disadvantage aka "mashing".
While this is risky and has a negative connotation, there is a time and a place to do this.
Let's look at a common case: while blocking the opponent's blockstring. Blockstrings often have frame traps, so using abare will lead to you getting hit. However, if they go for a standing overhead (which are usually slow), then you can hit them out of it by using abare.
Another common case is when the opponent attempts to tick throw you. The same principle applies - the opponent wants you to keep blocking so they can go for a throw. However going for a throw will lose to abare since throws need to wait for both blockstun and throw protection to end.
When to Do It?
As a general rule, if you think the opponent will go for a slow action (ex: standing overhead, waiting for blockstun to end and throw, running/jumping in and restarting a blockstring), then that is your time to strike. To further complicate matters the opponent isn't required to do one of these slower actions - they could just keep doing frame traps.
Thus successfully using abare is risky; finding the right time is difficult since opponent has a lot options, and doing it at the wrong time will lead to you taking lots of damage.