Simply put, advantage or disadvantage is a state imposed on a d20 check -- you either have advantage, disadvantage, or neither and it's a single die throw.
If you have advantage, roll a d20 twice for the check, and take the better number (and then add any other modifiers).
If you have disadvantage, roll a d20 twice for the check, and take the worse number (and then add any other modifiers).
Disadvantage and Advantage cancel one another out. If you have both advantage (NO MATTER HOW MANY CONDITIONS IMPOSE IT) and disadvantage (NO MATTER HOW MANY CONDITIONS IMPOSE IT) then they cancel out and you make a single d20 roll (and then add any other modifiers).
Its purpose as a tool is to make it easier for a DM to give a bonus or penalty without having to fiddle with tons of modifiers.
Example: I have two people trying to lift a heavy log in tandem. The DM decides that this equates to the Help action, and one player rolls the STR check with advantage. They roll 2d20, take the best result, and add their STR modifier.
Example: I have a fighter in chainmail trying to skulk around a quiet warehouse. Chain Mail armor automatically imposes disadvantage. The fighter rolls 2d20, takes the lower number, and adds their DEX modifier.
Example: The fighter is hanging for dear life off a cliff. The cliff is slick (DM rules it imposes disadvantage) but they have a buddy who isn't very strong but is braced and trying to lend them a hand to climb up (They are using the Help action). Because the climb has both advantage and disadvantage, it's instead a single d20 roll plus modifiers.
Make more sense?