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A 1-rep max refers to the maximum amount of weight you can lift for one repetition of a given exercise.

It’s used as a marker of overall strength and power in the exercise. It can also help you estimate how much weight to lift in the future based on the desired number of repetitions.

For example, if you want to challenge yourself with 5 reps of back squats and you know your 1-rep max number for this exercise, you can simply calculate the weight to put on the barbell as 85–90% of your 1-rep max.

According to a review from 2020, finding your 1-rep max is a reliable test of overall muscular strength. This applies to different age groups, males and females, and single-joint or multi-joint exercises.

SOURCE:

https://www.healthline.com/health/fitness/one-rep-max-how-to-calculate-and-use#what-it-is