The address lines are the bits that make up the address you are accessing. In order to access the RAM the system checks that certain bits in the address are low (0) to determine you are accessing the memory region between $0000 and $2000. This means you have less bits to check than specifically checking for less than $0800.
So what happens is if the address is $1FFF which is
Code:
01111111111111
32109876543210 - guide
That 0 which would start being set at address $2000 is letting the logic know to enable the RAM chip. Since the address lines above $7FF don't actually connect to the memory it loops around as the lines that are connected still change.
Not sure how to make it much more clear.
In a smaller example say you had a memory that was only 4 bytes. If the address is 00, 01, 10, or 11 they would all access a different byte of memory. When you try to access 100, that first 1 is ignored cause it connects to nothing. So you're accessing the same as 00.
Logically you can get the RAM address for NES by doing Address & $7FF. This logical and operation will remove any address bits higher than what actually exists on the NES RAM chip. I hope this helped.