The 38th parallel north is an imaginary line that goes around the Earth. It's like drawing a line around a big ball to show where it stops being one thing and starts being another thing.
This particular line runs across Asia, specifically through two countries called North and South Korea.
A while ago, North and South Korea went to war and couldn't agree on who should control the whole country. So they made a deal to split the country into two parts, with the 38th parallel north being the line that separates them.
Nowadays, North Korea is like one team playing on one side of the line, and South Korea is like another team playing on the other side. They don't agree on lots of things, but they try to stay peaceful by not crossing the 38th parallel north.