Messenger Lite is a slimmed-down version of
Not much to it -- but that's the point
This version is aimed squarely at markets where older mobile devices are common, particularly in the developing world, as part of Facebook's ongoing campaign to make inroads into what it calls "the next billion," internet users around the world who lack unlimited data connections and high-end devices.
As you might expect, Messenger Lite is a lighter version of Messenger. It allows you to do the basic functions -- send and receive messages with other users. Lite users can also send links and photographs, as well as receive stickers. In terms of features, it's a few years behind its larger counterpart, but it still covers the main functions of a messaging app. It's also much smaller and less demanding than the main Messenger app.
Easy to use, but maybe not for you
If you have a modern mobile device and access to a reliable, low-cost internet connection, there's no real reason to choose Messenger Lite. However, if you have an older phone or want to minimize your data use while still staying in touch with other Messenger users, this is an excellent way to do it.