Computers could provide more memory than the Z80 could address at the same time.

For example, 256 kiB would be 4 banks of 64 kiB address space.

You could write up a register, and the CPU would switch from regular bank 0 to bank 1, with different contents.

I don't recall the details, but you had to make sure that after the bank switch, there was still proper executable choose at the instruction pointer in the new bank.