'att' stands for 'attempts'. You might see this in a stats line for a quarterback, which would mean pass attempts, or in a stats line for a placekicker, which would mean field goal attempts or point after touchdown attempts.
attempt for what
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It means "attempts." A lot of NFL stats are percentages. You divide the number of times the player or team completed the task in question - passes, sacks, whatever - into the number of times they attempted to do it.