If the Chargers Win: They will remain all alone in first place of the division. They'll have a 2-0 record against divisional opponents (and 3-0 against AFC opponents), both wins coming on the road.
If the Chargers Lose: They'll be in a 3-way tie for first place in the AFC West with the Chiefs and the winner of today's Raiders-Broncos game. They'll have a 1-1 record against divisional opponents, which puts them a step behind the Chiefs (who would be 1-0) and maybe the Broncos (who could be 1-0).
In a nutshell, a win today puts the Chargers in complete control of the AFC West. A loss lets the Chiefs, Raiders and Broncos catch back up and gives everyone (just about) equal chance to win the division. A very important game.