Training to failure means doing reps until you cannot complete another one with good form. It can be effective because it guarantees high effort, and high effort is one of the biggest drivers of progress.
The catch is fatigue. Going to failure on every set of every lift will crush recovery for most people. That means lower performance in later sets, lower quality sessions later in the week, and eventually slower progress.
A better way is to use failure strategically. Isolation exercises like curls, lateral raises, leg extensions, and cable pushdowns are usually safer places to push all the way. Big compound lifts like squats and deadlifts are often better kept at one to three reps in reserve.
If your form gets messy at the end of the set, that is usually your stop sign. Technical failure comes before true muscular failure on many compound movements. Respect that difference. Grinding ugly reps just to say you failed is a great way to irritate joints.
A simple setup is this: keep most sets hard but controlled, then take the final set of selected exercises to failure. You get the upside of high effort without turning every session into a recovery problem.
Failure training is a tool, not a religion. Use it where it pays off and skip it where the risk-to-reward is bad.