You need to JOIN the two tables.
Here is a very good explanation: PostgreSQL - JOINS - Tutorialspoint
However, your tables need a column that is used for the join -- a common column (primary key in master and foreign key in the other).
For example you would have a column in your user table named quest_id.
Then you can :
select * user join quest on user.quest_id = quest.quest_id
Also normally you name your ids just ID in the target table.
For example your User table would have an ID column (not user_id) and your quest table would have a ID column (not quest_id).
Then you join might look like:
select * from user join quest on user.quest_id = quest.id
This will give all columns
If you only want one column you do the same query but list the only column you want.
select username, [description of quest] from user join quest on user.quest_id = quest.id