Neither of these teams will have been particularly pleased with their opening games of this World Cup.
Germany lost to Venezuela, 2-0, which immediately puts it under some pressure: it could be all but eliminated from the tournament if it loses to Mexico and Venezuela beats Vanuatu.
Of course, Germany will still expect to beat Vanuatu by a lot of goals, and three points plus a healthy goal difference should be sufficient to clinch one of the four knockout-round places available to third-placed teams from the group stage. But the Germans were lucky-loser qualifiers for this tournament in the first place: the team is leaving it very late to find form if it also needs a consolation-prize entry to the latter stages of this competition as well. So Germany could use a win, mostly to reassure itself that it has a shot at making an impact on this World Cup.
Mexico already has three points and would qualify for the knockout rounds with a win - since it is expected Venezuela will beat Vanuatu in the game that precedes this one. A draw would also make El Tri a likely knockout-round qualifier.
But a loss would complicate things for Mexico, because of the expectation that Germany and Venezuela will both beat Vanuatu. El Tri's problem is it didn't beat Vanuatu by much, so unless the Oceania qualifier proves as difficult to put away for the other teams in Group B as Mexico found it to be, chances are Mexico's goal difference is going to be substantially worse than that of its rivals. Worse, its goal difference isn't likely to be much more than ordinary.
Three points tends to be enough to get at least one third-placed team through in the format used by this tournament. But it might need to be three points and a superior goal difference. Being grouped with Vanuatu made the third-placed team in Group B look a certainty to make the knockout rounds. But since Mexico only beat Vanuatu by one goal, that calculation doesn't seem to apply: if El Tri is third in Group B, it might even have a negative GD.
There is room for a slip-up in this World Cup, but Mexico used its lifeline on a narrow win over Vanuatu. For peace of mind, it can't really afford to lose this game.
Per livesoccertv.com, here's where to watch the game: