No idea why you're being downvoted for asking a simple question about an acronym. From Wikipedia [1]:

ASDF (Another System Definition Facility) is a package format and a build tool for Common Lisp libraries. It is analogous to tools such as Make and Ant.

Contemporary developers using more mainstream languages are likely more familiar with asdf [2], the "Multiple Runtime Version Manager".

[1] https://en.wikipedia.org/wiki/Another_System_Definition_Faci...

[2] https://asdf-vm.com/