cave in = give way to a lot of pressure The pressure can be physical, or mental, like in an argument. I think the rest is obvious.

It can mean a couple things depending on the use. A tunnel can cave-in. For example: "The prisoners of war used wood planks in their escape tunnel to prevent a cave-in." But it can be used to express "collapsing." So, if a marriage is caving-in, it's falling apart and there will probably be a divorce. Another use is: "I'm not going to cave-in to all these troubles I'm having." It means your're not going to fall apart and stay strong in refusing to "cave-in" to life's troubles.

In this context I believe you intend it to mean "acquiesce" or "relent". For example, "I caved in and agreed to visit my in-laws for the holidays".