Okay the short answer to make it really simple is like the Unimind from Buzz Lightyear of Star Command where Zurg imposed his mind/will across the galaxy.

The long in-canon answer is that it is a meta-mathematical formula that if solved and understood, basically allows the user to "hack the source code of the universe" in which they could assume control of every living thing that exists both mind and will. Darkseid is the god of tyranny, and basic conquest is inefficient. He wants the Anti-Life Equation to impose his will and rule across the universe in one fell swoop