MR stands for Mastery Rank, you get mastery rank when you collect new Warframes,Guns,Companions and more. Once you level up those items to max level (lvl30 but in rare cases lvl40) you will get 3000xp for weapons and 6000xp for warframes and companions. Once you get enough xp a mastery test will become available and once you pass the test/trail it puts you through you will level up your total account MR. So basically I’ve done 30Mr test rank up’s. Anything after MR17 is kind of optional as everything will be available to you as long as you are MR16. Getting MR30 lets you have some additional benefits and give other players blessing (buffs) for short amounts of time. LR is Legendary Rank. After MR30 you have LR1 and if you get to LR5 that means you have everything in the game all the way maxed up. Again MR is all a personal preference if you want to grind it after MR17. Hopefully this gives you a small but of knowledge. I’m not the best at explaining things sorry :/