A trial by fire is a medieval practice of setting someone on fire (or asking someone to walk through flames) and judging that they are innocent if they survive the ordeal, and guilty if they aren't.
While the practice itself is barbaric and frankly, quite stupid, and has become extinct a long time ago, the phrase has stuck on into current times. Today, trial by fire simply means putting someone under a lot of pressure to see how they perform.
Trial by fire is also an ultra-learning method. For instance, forcing yourself to speak only in a new language for three months until you learn it and become comfortable speaking it is a trial by fire.
Some things, especially something like learning to speak a new language, is learnt better via a trial by fire method.
If you're setting out to learn something new, you might want to put yourself under pressure to do so, rather than simply go over the theory.
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