Accessing RAM on TERASIC DE0 Nano
The address bus is used for either column or row address, depending on the control signals (RAS/CAS and friends).
I don't know which SDRAM chip is on your board, but they are pretty much standardized these days, and differ mostly in timing and maybe in burst options.

Here is a brief overview: Understanding DRAM Operation.
Having said that, definitely find the datasheet for your SDRAM chip - it's very informative!

E.g., my DE10 Lite board has this chip: iS42/45S16320F-7TL.

Yours probably has 12-wide row address, 10-wide column address, in total giving  \$2^{12+10+2}\$ = 16M words, which for 16-wide data means 32MB (not 32Mb).