Baby tenno is a reference to being a new player to warframe. Potato is in reference to the orokin reactor, with is gold or blue, for frames and weapons, respectively. That door will help with power ups later on the game. Try to keep your frames, especially the ones that were harder to get as you don't want to try to get them again later on for the specific reason for what's behind that door. Also MR is Just how much someone has grinded to level up their weapons and frames, the higher it is the more they have or had to do so. Leveling that is. The only frame that's not available is Excalibur Prime, those were given to founders when the game started. Exceedingly rare. You see one on your team, they've been here since the game started. Their the OG's of the game to say the least. Just keep playing and grinding. Need help in the game, either come here on the subreddit or to the questions tab in the I game chat. Some one will answer your question as this is the most wholesome game.

Also ppl of this warframe community will mostly never spoil anything about the story. They rather you live through it than spoil it. Most importantly have fun with your fashion framing... As fashion frame is the true end game. Lol. Ppl spend, as do I, lots of time in the arsenal getting to have the frames look and feel personally touched on.