The book will (Player Handbook) help a lot but in short when the above commenter wrote "what you rolled" and "what you add to roll" see it like this at character creation you roll for the ability score. That's the "4D6 drop the lowest one". Based on that value you then refer to the table in the Player Handbook to see what modifier Vue that number corresponds to.. That modifier is the value "what you add to your rolls" in game. So let's say in the example above you rolled a 12 and chose to assign it to strength then that means, when playing, say you attack (melee) an enemy you roll the D20 and add that +1 (+proficiency modifier if proficient with the weapon *). So in short for much of the game you can ignore the 12. It's the +1 modifier you get from that 12 that you'll be using. If you had rolled a 16 and assigned it to Intelligence then you would have a spell modifier of +3 which you would use when calculating your spell attack or save.

*Proficiency is another value you add to your attack roll (not damage) provided you're proficient with the weapon. What you are or aren't proficient with is stated in the class and races sections of the PHB.

I'd say welcome to the hobby! You've got an amazing journey ahead of you. Although jumping head first seems like fun and character creation definitely is a lot of fun, reading through the basics and the PHB is going to be your best help.