Demaryius Thomas of the Denver Broncos passed away last week. I didn't know him when he was playing so I feel I would be doing him a disservice by trying to describe him to you. Instead I will refer you to an article he wrote about his early life. Warning: you may cry while reading it.
To mourn the loss of one of their own, a football team will line up in "missing man position". It's like a normal lineup except there's only 10 men on their side. They deliberately leave the position of the deceased empty. If the mourning team is on offense, they'll then commit a delay of game penalty which they expect the opposing coach to decline.
This is what the Broncos did on Sunday. On the first play they left his position, wide receiver 1, empty. The Lions defense spotted this, but since this had been pre-arranged between the teams, they unset themselves and walked up to the line. Delay of game was called, the Lions coach declined, and everyone clapped and possibly cried.
This does not always go to plan and it is harder to control if the deceased was a defensive player. A few weeks ago, Washington lined up in missing man to mourn Sean Taylor. The Bills were not told about this, did not spot it, threw a pass to where he would have been, and gained about 40 yards. Were they told of this in advance, they would have taken the delay of game and Washington's coach would have declined it.
This is not one of those hard and fast rules but a common courtesy among teams on both the college and professional levels. There are coaches who would take the penalty or play the down. Two such coaches that might do such things would be noted massive assholes Matt Patricia and Urban Meyer. Someone suggested that Bill Belichick might be another but I don't think that's the case. He may be demanding, a perfectionist, and sometimes just barely on the fair side of the rules, but he's not that level of scumbag.