@Dreikin correct.
To explain a bit more thoroughly, PHP has a setting that sets a "soft" limit of memory that scripts are allowed to consume.
In current versions the default is 128M, meaning that for every script that runs (= one page being served, typically), that script could theoretically consume 128M of memory, and trying to allocate above that results in a fatal error. This is still subject to the system physically having enough memory to give out to the script; you can't just say "let my script consume 100GB" on a machine that doesn't have it - the alloc request will fail and you get the real "Out of memory" error.
So, at some point in the lifetime of that script, either at config time or during runtime, the limit was upped to 2G, so that the script could allocate anything up to that but not beyond.
It's really just a safety measure to prevent runaway scripts eating all the memory. But 2G is a stupid high number, even for a platform that flat out asks you to up the numbers from the defaults - the default is 128M, last I checked Magento they insisted on 256M minimum, 512M recommended... And this is the bottom limit to serve any page on the platform.