MLT = Mash/Lauter Tun
This is referred to as such when it is the vessel you would perform the mash and also sparging (rinsing) the grains.
HLT = Hot Liquor tank
I think its only necessary if you are going to fly sparge. Batch sparging requires no such vessel. It's just to hold hot water for sparging.
Both of these pieces of equipment are for all-grain brewing where you start with the grains, mash (let rest in 150-158 degree farenheit water for an hour), then separate the sugary liquid from the grains (called wort).
I think most beginning brewers start with extract brewing where you would not perform a mash and thus, not need the MLT or HLT. Some people do start with all-grain brewing. It just requires more time and additional equipment. I would recommend getting a BK of 10 gallons and start with full boil extract batches. Then if you decide you really do enjoy it as a hobby/obsession, you can always add an MLT (and a HLT if you want to fly sparge).
Also, check out youtube videos on homebrewing. I also recommend reading How to Brew by John Palmer.
Welcome to the community and homebrewing. Don't be afraid to ask these questions.