What is Head-to-head rule in football?

Answer 1

In the group stages if two teams are tied in points then their position is decided by the result of the match in which they played each other. The winner of that match will take the higher of the two places regardless of goal difference. Goal difference will only become relevant if the two teams draw.

e.g. Group A

After 3 matches Greece and Russia were tied on 4 points

Russia had a goal higher goal difference(+2 to Greece's 0) but Greece took the higher spot because they beat Russia.

Answer 2

the result in the game between two teams.

The teams having equal points. And who ever won the match between those two teams

Qualifies.