Sometimes the winner of a game seems obvious. Like when the USA dream team played Angola in the 1992 Olympics. So instead of picking who will win the game outright, odds makers create a handicap for one team, which is referred to as the line (or spread). So instead of picking USA to win vs Angola, you'd pick who would win if USA had a handicap of -40 points.
The goal of an over/under pick is to guess the total number of points that will be scored in a game. In the past few decades, offenses have been getting better. In the NFL in 2020 teams averaged 24.8 points per game.
You might not be so sure about some picks each week. Or you might not care about certain games or teams. No problem! Just put those picks at the bottom of your list and they will have a marginal impact on your weekly score.