I'll have a go - as a complete statto, as far as I'm concerned this uber-complicated **** is 100 times better than pointless friendlies!
All the nations in Europe are being divided up according to their UEFA coefficients into four pots - the best in Pot A, the hopeless minnows into Pot D.
Each of those pots is then divided up into four groups. The teams in those groups play each other home and away between the World Cup and Euro 2020. Group winners of each group in Pot A go onto a semi-finals and finals to be played in summer 2019, with the winner getting a tinpot trophy. For Pots B-D, those groups winners get promoted - so in the next tournament (between Euro 2020 and the World Cup 2022), they're playing in a higher Pot. Conversely the teams that finish bottom in each group in Pots A-C get relegated for the next tournament (with one minor wrinkle in Pot C that I'll ignore for now).
So far, so simple.
Where it gets a bit complicated is that they're also using this as a way of qualifying into Euro 2020. Each Pot will also be guaranteed one qualifying place in that competition. The way they're deciding that is waiting until after all the Euro qualifiers are finished, and then having the four highest ranked teams that didn't qualify for Euro 2020 in each Pot playoff against one another for a spot. If there aren't enough teams, they add one from a lower pot to make up the numbers.
This will have two effects - one, it'll mean that a team from Pot D i.e. a complete minnow will definitely qualify for the Euros. Two, it gives the 'big' nations a second chance to qualify if they **** up the first time (despite the fact that with 24 spots, it should be easy - looking at you, the Netherlands!)