An address is a unique memory location within the flash memory of the device. The address range starts from 0 and increments by 1 for each successive memory location within the device.
At a very simple level, if you were to write your 50 byte string starting at address 0, then that would write the characters into addresses 0 to 49. Your next string would then be written to addresses 50 to 99 and so on.
There's a bit more to it than that - but that's the basics.