In common lisp, you don't need a build system at all; you can `(load "file.lisp")` everything and it should generally just work. But of course, build systems are useful tools, so nonetheless ASDF exists and it's nice enough to the degree that nobody has built a better and more widespread common lisp build system.
Some good trivial examples are in the lisp cookbook:
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".
