Your mmr is higher than your actual rank if you are playing with golds but are actually silver. If you win those games you get lots of lp. If you lose them your mmr will drop back to silver and you'll be at square 1, and the rank you deserve
So to get an idea of MMR, you need to think of your metal+number rank as your raw amount of lp above iron 4 0lp, so if you were iron 3 24lp, you'd say you had 124 lp, if you were gold 3 56lp, you'd say you had 1356lp.
Think of your MMR as some other number of lp on the same scale. Now here's a formula:
LPG-LPL is proportional to MMR-Rank
Where LPG is lp gained on your metal+number rank if you win, and LPL is lp lost if you lose. So if your rank is way higher than your MMR - you as a silver matches with bronze or something - and you lose those games, you lose A LOT of lp, and if you win, you gain very little.
So the way you climb out of silver is to prove that you can go toe to toe with gold players. You've obviously been winning more than losing if your mmr is gold, so your mmr has climbed. Now the game puts the ball in your court: are you gold?
Think of your MMR as something that is supposed to challenge you. If you go on a loss streak, the game will say "can you influence this lower ranked game enough to prove you don't belong in this rank"
If you win a lot and get placed in a higher ranked game, the game is saying "Do you deserve a higher rank?"
Hope this helps
