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A one-rep max is the heaviest weight you could lift once. Rather than testing it — which is slow and risky on your own — this estimates it from a set you have already done, and turns it into the training percentages a programme is written in.
Two formulas are shown. Epley multiplies the load by one plus the reps divided by thirty; Brzycki divides the load times 36 by 37 minus the reps. They disagree by a few percent, and seeing both is more honest about the precision than a single averaged figure would be.
Both formulas assume a set taken close to failure with clean technique, and both drift as the rep count rises — which is why this stops at twelve. Never treat the estimate as a weight to attempt without a spotter.