Percentages will be used for at least 2 different scenarios. One will be when describing a percentage of your 1 RM. When you see: 3×5 @ 75%, this means 3 sets of 5 reps at 75% of your 1 RM. Another use will be when describing effort. When you see, for example: 1 km row at 90% it means row at only 90% effort, don’t set out to go for a PR.