Before running a protocol, when you're in the Calibrate tab, you might see this warning.
The Opentrons App shows you this warning if you try to recalibrate tip length when that tip already has a tip length calibration.
You shouldn't need to recalibrate tip length unless you suspect something went wrong the first time.
Plus, depending on the model of tip, recalibrating its tip length might also force you to recalibrate your pipette's pipette offset. This will happen if you performed that pipette offset calibration with that model of tip. For more details, see How positional calibration works on the OT-2.
If you think something is wrong with your OT-2's calibration, see Troubleshooting movement, positioning, and calibration problems, or contact Opentrons Support.