It's nothing to freak out about yet girl. TRY not to stress! I know it's SO hard. My kiddo was diagnosed with the same thing when I was pregnant and I panicked. Like almost called off his babyshower cause I thought somethin horrible was gonna happen. They will just start monitoring you more often, more ultrasounds and checkin up on babe a LOT more often. It just means he's not growing as fast as they'd like. In my case, it was just because I'm a pretty small framed person and he just didn't have the room. We went to weekly ultrasounds with the worry of a possible early delivery. They'll just watch you guys and if it gets to a point that baby just can't grow inside anymore, they'll induce you. Their goal, OF COURSE, is to get you as close to full term as they can without presenting a danger to baby or Mama. It's just gonna take time to monitor. TRY to relax and just take it an ultrasound at a time girl. Also ask ALL and ANY questions that come to your mind when your in a check up.
It means the baby is isn't growing at the rate expected. It's not anything bad news just right off the bat perse, but it is something that will have to be monitored.