"Hey, come on, we know you are better than this. Let's get it together."
If we didn't think you were capable of playing better, then we would just sit down in disappointment that we don't have a better team.
Let's have a little perspective. There are 50,000 people who are no closer than 30 feet from you. They want to convey a message to you, en masse.
When they think you are doing well, they cheer, rather than try and synchronize a chant of, "That was an amazing play. Great blocking. Great athleticism. Thank you for making us happy."
When they think you can do better, they boo, because it is impossible to "talk" about our feelings and how we know you can do better in that setting.