I think it's when you pull LO up from a laying position by his/her arms, and the neck doesn't work to support the head so the head "lags", or falls, backwards. That's my guess anyway.
No advice for OP except to work on tummy time like PP said.
My LO was born 1 day before yours and still has a bit of head lag, and he's still not bearing weight on his legs. A pediatrician saw him last week and reassured me that his muscle tone is great, and nothing is wrong. It set my mind at ease. Mention it to yours, for the peace of mind more than anything else.
Most babies are ahead on some milestones and behind on others. There's a range of what "normal" is. Try not to compare!
My lo was born June 7th... She still has a slight head lag as well as torticollis. We are currently going to physical therapy for the torticollis & other then a brief mention, the therapist has not brought up the lag. Like everyone else said, bring it up to your dr and try to do more tummy time
Re: Head lag
No advice for OP except to work on tummy time like PP said.
Most babies are ahead on some milestones and behind on others. There's a range of what "normal" is. Try not to compare!
While making this leap, you will also notice that:
Your baby's head has to be supported again more often;
As the others said, it can't hurt to talk with your doctor too.