“You'd really better follow through.”

Your husband was invited to meet his friends at a bar for drinks and told his friends that he would go. But now he doesn't want to go and is considering staying home instead. You think he should go because he told the friends that he would. You

If you promised them you'd be there, you'd really better follow through.

promise (someone)(clause)

A promise is a definite, formal statement that you are going to do something. This phrase describes what was promised and who the promise was made to:

She promised us she wouldn't do that any more.

I promise you that I will never do that again.

Using "that" before the clause makes the statement more formal.

I'll be there

This means that you will go to an event that you've been invited to:

A: Are you coming over later?

B: Yeah, I'll be there.

you'd better (do something)

This is a strong way of giving someone advice.

You might expect that adding "really" to this phrase would make it stronger, but actually it makes it sound a little softer and more polite.

follow through

This means to do what you said you'd do, or do what you promised.

You can express what you're following through by using "follow through on ___":

Are you going to follow through on your promise?

You can then express what the promise was using "(someone's) promise to ___"

I'm starting to think that she might not follow through on her promise to pay us back.