The heaviest weight a lifter can move for a single rep through the full range of motion with acceptable technique.
It is used two ways: as a test of strength, and as the reference number for percentage-based programming, where "5 x 3 at 85%" means 85% of the 1RM.
Testing it means warming up to heavy singles with three to five minutes of rest and stopping at the first rep where technique breaks. That is a risk for beginners, so most trainers estimate instead. The Epley formula is 1RM = weight x (1 + reps / 30): a set of 5 at 100 kg gives about 117 kg. Estimates are reasonable up to about 10 reps and unreliable beyond that.
Once you have the number, 70 to 80% covers sets of 5 to 8, and 85% and above covers sets of 1 to 3. Re-test or re-estimate every block; a 1RM from three months ago is a different person's number.