An "issue" is a problem:

If there are any issues that require immediate attention, I can be reached on my mobile at 646-469-XXXX.

Use the phrase "The issue is..." to explain a problem:

A: If you're too busy, why don't you hire someone to help you?

B: Yeah, I've thought about that. The issue is that it would take so long to train a new person, and I just don't have time to do that.

You follow "The issue is..." with a noun or noun phrase:

The issue is Kana's school. They won't let her take that much time off.

The issue is how we're going to get all of those people from one location to another.

This phrase is common in business situations and slightly formal situations.