Modification vs. Enhancement
What it is
A modification changes SAP's own delivered objects (a repair); an enhancement adds your logic at SAP-provided extension points (BAdIs, enhancement spots) without touching the original. Clean Core forbids the former and governs the latter.
Why it matters for Clean Core
Modifications are the original sin of upgrade pain — every one becomes a SPAU/SPDD conflict on the next upgrade. Enhancements through released extension points are upgrade-safe by contract.
Common pitfall
Believing an 'access key' modification is fine because 'it's only a small change'. There are no small modifications at upgrade time. If you're editing an SAP object, you're on the wrong side of Clean Core.