“(Clear your throat) Can I have everyone's attention for a moment?”
You organized a going-away party at a restaurant for your friend who's moving to another area of the country. Everyone has been eating and talking, but you want to say some nice words about your friend to the whole group. To get everyone's attention, you say this.
(Clear your throat) Can I have everyone's attention for a moment?
When English speakers want someone to pay attention to them, they make a sound that we call "clearing your throat." Sometimes in stories it's written as "Ahem".
It's hard to explain the sound, but basically you make two noises with your throat. The first one is a short grunting sound, and the second is a little bit longer and lets the air out of your lungs.
Say this when you want to speak to a large group of people. When you say this, it means that you want everyone to be quiet and listen to you. After that, you usually make an announcement, propose a toast, or give a short speech. You can use this at a party, at home when you have something important to tell your family, or at work in a meeting when you want to ask everyone something before the meeting starts.
There's a similar phrase that you say when you're making an announcement over a speaker or intercom, like in a school or on a train:
May I have your attention please?
"A moment" is a short period of time. It's used to talk about something that doesn't take long. For example:
There's no specific number of seconds that "a moment" refers to. It can mean anywhere from 15 seconds to 5 minutes. You can also say "for a second":
Can I have your attention for just a second?