“I wish you'd told me sooner.”

Your department is working on a long-term project that is supposed to be completed two days from now. One of your employees comes to your office to tell you that there is a problem, and he thinks the project will be delayed an extra 3 or 4 days. You are angry because you think that someone should have warned you of the delay before this point. You tell him:

I wish you'd told me sooner.

I wish (something) had (happened)

This phrase is used as a polite way to complain to someone about the way that something was done. "I wish" is a way to talk about the thinks that you want but aren't possible or likely. When you use "I wish" to talk about the past, you are saying the way that you would have wanted something to be done.

"I wish you'd ___" is more polite-sounding, but at the same time more disappointed and critical-sounding than "Why didn't you ___?":

Why didn't you tell me sooner?

In spoken English, it's more common to shorten "you had" to "you'd", "he had" to "he'd", and so on.

tell (someone)

The word "tell" is always connected to two pieces of information: who you're telling something to, and what you're telling that person:

I wish you'd told me that you were leaving sooner.

In the example at the top, one of those is answered ("told me"). But the other part is understood from the context. Both people understand that the message is "the project is going to be delayed".