Attack Roll Question

I was playing the other day and came across this and didn't quite understand. Why is it when I click on Eldritch blast, it shows two numbers? I thought you roll one d20 to see if it hits. Then click on eldritch blast wording to roll the damage roll.

I think it is for when you are at an advantage or at a disadvantage. I'm guessing you are using the 5e system and they have that mechanic for combat rolls.

It's so that if you had advantage or disadvantage and forgot before making the attack roll, you still have the second roll just in case. Normally, you just use the left roll. If you have advantage, take the higher. Disadvantage, take the lower. You can change whether you always roll two or something else in the top right corner by clicking on the gear icon and reading through the options below it on the right side column.

Thank you! This makes perfect sense! :D