Warning: SPOILERS ahead for the first act of Avengers: Infinity War
Avengers: Infinity War brings together almost all of the Marvel Cinematic Universe's biggest superheroes for the fight against Thanos, but Ant-Man and the Wasp aren't on the team this time. The shrinking duo will be seen in their own solo movie, Ant-Man and the Wasp, later this summer. As we learn in a brief bit of exposition, Scott Lang sits out the Infinity War because he's on house arrest as a result of his role in Captain America: Civil War.
Unlike Civil War, in which the Avengers on both teams had a few opportunities to stop, catch their breath, and call in their allies, the events of Infinity War unfold at a breakneck pace, with the entire movie taking place in roughly the span of a single day. After Thanos' initial attack, Bruce Banner returns to Earth to warn Doctor Strange and Iron Man, and barely has time to explain what's happening before Thanos arrives on Earth. Iron Man, Doctor Strange and Spider-Man are all whisked away on Thanos' ship, while the Black Order attack Vision and Scarlet Witch in Scotland. Captain America, Black Widow and Falcon come to the rescue, and then all five rendezvous with War Machine and Bruce at Avengers HQ before heading off to Wakanda in a race against time to separate Vision from the Mind Stone before Thanos' lieutenants attack again.
What all that means is that there isn't a whole lot of time to call Ant-Man in from San Francisco, and as far as we know the Avengers don't even know who Wasp is (let's be honest, they barely know who Ant-Man is). However, while the team are at Avengers HQ it is explained that both Ant-Man and Hawkeye cut a deal with the government after their escape from the Raft, allowing them to return home to their families on the condition that they remain on house arrest. While that's a somewhat weak excuse for not calling in Ant-Man when the lives of half the people in the universe are at stake, it's serviceable enough. The real reason, of course, probably has more to do with the writers (Christopher Markus and Stephen McFeely) and directors (Anthony and Joe Russo) wanting to save Ant-Man and Wasp for Avengers 4.
As for what Scott Lang and Hope van Dyne are actually doing during the events of Avengers: Infinity War... well, we already know that Scott will have a hard time sticking to his house arrest. The trailer for Ant-Man and the Wasp showed him being fitted with an ankle monitor, but his mentor is a guy who invented shrinking technology, so thwarting an ankle monitor probably won't pose much of a challenge. In a voiceover, Hope says, "Thanks to you, we had to run. We're still running," which could mean that she and her father were targeted by the FBI after Scott's Civil War stunts revealed his shrinking/growing tech to the world, or could be the result of something that Scott does in the movie. Either way, they're on the lam.
The important thing to know about Ant-Man and the Wasp is that it's set before the events of Infinity War, so don't expect to see Scott and Hope trying to deal with the fallout of the Avengers' tousle with Thanos. We also don't know where Ant-Man and Wasp will be at the end of the movie - who knows, perhaps they sat out the Infinity War because they're stuck in the Quantum Realm. What we do know is that the original Wasp, Janet van Dyne (played by Michelle Pfeiffer) is set to appear in Avengers 4, along with original Ant-Man Hank Pym and (presumably) the current Ant-Man and Wasp. At that point, we'll probably find out exactly what they were up to while the Avengers were trying to save the universe.