That's called the CMOS battery, and maintains your BIOS settings and system clock. If it's dead, your computer clock will need to be reset after a cold boot, and if you changed any BIOS settings from default those changes will need to be remade. It used to be much more important back when you had to manually program your hard disk settings and CPU settings and memory timings and card interrupts in order to boot. That's all automatic now, and if it's network connected the OS can set the clock automatically, too, so the CMOS is basically redundant if you just use standard settings.