I'm still making my way through Undermountain, but I saw a great description of what you can expect to do when you are done!
How do you start doing Master Expeditions?
Its the last thing in the Undermountain Campaign. Once you finish it you can do 3 ME's a day.
The flow of Undermountain end-game goes like this.
Finish the campaign, that unlocks Master Expedition dailies as well as a quest to unlock Resonance. Both quests are on the top back/left side of the Yawning Portal.
Once you do the resonance quest, basically when you're not doing your 3 daily ME's (Master Expeditions) you will do heroic encounters in any of the Undermountain zones. Kinda like demons in older zones.
Resonance drops into (believe money pouch?) doesnt really matter as you don't really touch them. Anyway, The resonance you can use to create crystals, these crystals are basically something you add to change the flow of how easy/hard the ME's are and the item/gear drops.
More detail: Know when you first go into even a leveling encounter there is a small cave room. To the right are small stairs leading to 3 stone slabs. IF you are the group leader of the ME, you can click on the first one. This will allow you to convert your resonance, into crystals.. Then you can click on each of the 3 stone slabs to add 1 to 3 crystals. Each crystal type does something different, like Chaos makes it really hard and the more crystals you add increases it.
Confused yet? Once you do it, its not so bad. If you did Barovia, think of resonance as a "card drop" and a crystal is the actual card you add to the "hunt". Stronger or higher cards made hunts give better drops. Same with crystals in Master Expeditions.
Personally I like to do 3 crystal ME's as you get a purple on almost every boss. But you only need 1 crystal to open the final chest. So people tend to form that kind of group to quickly grind their dailies.
BTW and this is a pet peeve of mine. Seems EVERYONE calls them "1 rune runs" or "how many runes do you want added" They are crystals! lol Runes are what you get from the final chest and they are called "runed etchings" these can be used to level artifact gear, but most people use them to buy keyless lootboxes from Zox. He's under the left stairs in Yawning Portal.
I love zox boxes
they give gold and AD and a chance at a high level gear piece and vanity pets
I'm up to 4 vanity pets from undermountain and even had a ilvl 1k headpeace that gives 2k power against bosses.