SRAM, in the context of Sonic hacking, stands for Saved RAM. Also known as "battery". This is usually small - about a kilobyte. As the name "battery" implies, SRAM must have power constantly running through it, or it will be erased, and the smaller it is, the longer it will last. A typical battery lasts around a hundred years, so there is no need to worry about them running out.
The term "SRAM" is sometimes used to refer to the System RAM, which is more commonly called simply the RAM.