Beta is climbing slang for the information needed to climb a route well — which holds to use, which feet to place, which moves to do in which order, and any tricks that make the climb easier. Climbers share beta with each other, debate beta, write beta in guidebooks, and watch beta videos online before trying hard projects.
The word came into climbing from rock climber Jack Mileski in the 1980s, reportedly named after the Betamax video tapes climbers were using to record route attempts. "Got the beta?" became shorthand for "do you know how to do it?" and stuck.
There's sometimes a distinction between "beta" (any information about the route) and "spray" (unsolicited advice, often unwelcome — "no spray bro"). Most climbers want beta from people they trust but appreciate the chance to figure out climbs on their own first. The phrase "no beta" or "spoiler-free" requests indicate the climber wants to onsight the route. Etiquette: ask before sharing beta with someone working a problem, and accept that different climbers may use different beta on the same climb.