Imagine a bunch of tiny invisible arrows that are always pointing towards a magnet. These arrows are called magnetic field lines. The magnetic flux density is like how many of these arrows there are in a certain area.
Think of it like a crowded room with a lot of people. If there are only a few people in the room, it's not very crowded. But if there are a lot of people, it's very crowded. The magnetic flux density works the same way. If there are only a few magnetic field lines in a certain area, the magnetic flux density is low. But if there are a lot of magnetic field lines in that area, the magnetic flux density is high.
Magnetic flux density is measured in units called teslas. So when someone talks about a magnet with a flux density of 1 tesla, it means that there are a lot of magnetic field lines in a certain area. And when someone talks about a magnet with a flux density of 0.5 teslas, it means that there are fewer magnetic field lines in that same area.