씨 is a respect suffix indicates that the person speaking has respect for the person they are talking about. It doesn't really have a direct translation into English but it is a sign of respect. It is attached to the names of people you have just met, acquaintances or even friends who aren't super close.
When talking to really close friends or children, the suffix 아 can be added.
Remember, do not address people who are older than you or who have a higher position than you at work with 씨. 씨 is often used among people of similar age who are not super close. You could use this word to refer to your friends, acquaintances, and work colleagues. You can also use 씨 with people that are younger than you. Always say the full name (or just the first name) of the person + 씨. Saying the first name + 씨 sounds more friendly, I think.