A clause (Most Favored Nation) that lets an early investor automatically inherit any better economic terms, like a lower cap or bigger discount , that a later investor negotiates in the same round.
When YC published the post-money SAFE in late 2018, it released four standard variants, one of which is the SAFE with MFN provision. The clause lets the SAFE holder amend their terms to match any subsequent SAFE the company issues before the priced round. It became the default tool for early angels who want to write a check without anchoring the price.