UEFA has system to pick teams from countries for group stages and which ones require to play qualifying round.
Those teams who awarded direct entry into group stage are who occupies top 3 league places in countries ranked one to three. Also the 1st and 2nd placed teams in countries which are ranked 4 to 12.
For those teams who are unable to qualify automatically will have two qualifying routes Champions route and League route.
Champions Route
First qualifying round meets champions of countries which are ranked Fifty to Fifty Three in UEFA competition play 2 two-legged ties. Two winners of ties proceed to 2nd qualifying round and joined by winners of thirty two countries which are ranked seventeen to Forty nine. The Winners from those 17 ties join winners from countries ranked 14 to 16 in 3rd qualifying round. Champions of these ten ties went to play off round. Champions of those 5 ties will reach group-stages of Champions League.
League Route
The 3rd placed team from 6th ranked member association begins in 3rd qualifying-round along-with runner up of the association’s ranked seven to fifteen.
Winners of those 5 ties went to the play off round where they will joined by 4th placed teams from member associations ranked one to three, and 3rd placed sides from the associations ranked four and five. Teams that become victorious from those 5 ties will go to group stages of Champions League.