The "It" in You Can't Take It With You is money. Money, money, and more money.
GRANDPA: You can't take it with you, Mr. Kirby. So what good is it?
Grandpa goes on to say that love is more important than money, and if Grandpa says it, who are we to argue? Still, it's worth pointing out that if money matters so little, it's odd to put it up there in the title. If no one cares… why care?
Of course, people do care. Grandpa, and the film, are trying to get them to care less. But money was on everyone's mind in 1938, which was in the middle of a pretty serious Great Depression. Many people didn't have work; many people didn't have money. So the title is presented, in the film, as a reprimand to rich people for hoarding wealth and working too much. But not-so-secretly, off to the side, it's reassuring people who don't have any wealth and are working too little.
So You Can't Take It With You means, on one hand, "Don't be a miserly jerk like Mr. Kirby." But on the other hand it means, "Don't worry if you aren't as rich and powerful as Mr. Kirby; you're still a good egg, as long as you've got friends—and a nice movie by Frank Capra."