Chritie is meeting Trump.
Probably asking for Trump to pay off his campaign debt.
He will then endorse Trump.
Carson will be meeting with Cruz tomorrow, probably a similar situation.
It is how the game is played.
Donation limits still apply, so a single person can’t pay off someone’s campaign debt. Trump could promote a fundraiser or appeal to his supporters or donors to give donations to Christie though. Cruz did this for Walker.
Carson’s meeting with Cruz is the one Cruz requested to discuss the Iowa situation. Carson has unfortunately said he’s staying in past South Carolina as long as donations and social media support continue. Actual votes aren’t a requirement apparently.