It means "I fully agree with you" or "I agree  with you completely".

Remember that agree is a verb, so "I am agree" is not possible. This is a common learner error.

"I am wondering what does this phrase mean?" is also a learner error. No native speaker talks like this. We say, "I am wondering what this phrase means" because it's a statement and not a question. Regular word order.

I don't think I've ever heard "I can't agree with you more".

The common phrase where I'm from is "I couldn't agree with you more".

• 黄晓君 above is correct. "I can't agree with you more" is short for "I cannot agree with you more," which suggests that speaker agrees so much with you that it would be impossible for them to agree more. It's a common saying in conversation.