New to D&amp;D and I need some help with something I noticed something on some actions when it comes to hits but I can't find any explanation for it even when looking through  the resource side bar...When it says when rolling for hits the dice to use and I understand that but a lot of the time I see stuff like "Hit: 7 (d8+3)"

What does that number in front of the brackets mean? Does it affect anything or add anything to the hit?