What is beta spray?

Beta spraying is when one climber tells another how a route is supposed to be climbed, or what they need to do, without them asking for the help.

This annoys many climbers for a few reasons.

First, a big part of climbing is the mental challenge of figuring out the puzzle.
Figuring out the beta on your own helps you progress as a climber by being able to understand the problems and coming up with solutions on your own. Being able to correctly read a route and to apply your own climbing style to it in order to complete that route is what can make the difference between you being a beginner climber to being a more advanced climber that can flash routes.
Without learning how to problem-solve and to read routes without being told the beta, you will find it extremely difficult to progress to a high level in climbing and to be able to flash routes without being told what to do. And those who beta spray sometimes take this opportunity away from others by giving them the answers and not giving them a chance to think about it and solve it on their own.

Second, the beta they give to others may just be wrong.
From my experience, those who beta spray are usually either beginner climbers, or, climbers that are slightly above the beginner level that feel comfortable enough to tell others how they should climb a route, either because they were able to do it, or because someone else told them what the beta is. Either way, the beta they give could just be wrong, and in many occasions, when this has happened to me, they were wrong.

Third, the beta could be correct, but for them.
Everyone has their own climbing style, so the solution that works for one climber, in this case, the beta sprayer, may not work for you. This is something that many climbers don't understand. When they are persistent in saying that you need to do it in a specific way, a way that maybe worked for them, sometimes, it just doesn't work like that for others.