The SE is the crapper version of the LG G5.
The main differences between the G5 and the G5 SE are internal downgrades, likely meant to lower the price for markets like Russia and Latin America:
No LTE support
Snapdragon 652 instead of Snapdragon 820
3GB of RAM instead of 4GB of RAM
Otherwise the two phones are nearly identical. Same design (although the G5 SE is 7.3mm thick compared to 7.7mm for the G5), same 5.3-inch Quad HD IPS display, same cameras, and even the same modular design. While it's unclear if LG will be bringing the G5 SE to the US, or any Western markets for that matter, you won't be missing out on much.
You'd think SE stood for special edition.....but probably not a bad phone at the end of the day...