If someone needs to see you and you have some work you need to finish urgently, use this phrase to say that you know someone is waiting for you and you will arrive as soon as possible. Typically, this can be used in a store when a client is requesting the attention of a salesperson who is currently busy with something or someone else.
Similarly, you can replace the word moment with the word minute. Or you can say you’ll be right with someone. The sentences would then look like this:
“I’ll be with you in (just) a minute.”
“I’ll be right with you.”
Good morning! I’ll be with you in a moment.