Imagine you are playing in a big field with your friends. Suddenly, someone drops a bunch of invisible toys that can hurt you very badly if you step on them. These toys are called landmines.
In Bosnia and Herzegovina, a long time ago, there was a bad war between different groups of people. During the war, lots of landmines were left in the ground and nobody cleaned them up after the war ended. So, now the toys (landmines) are still there and they are very dangerous.
Now, people who want to use the land for farming or building houses have to be very careful where they step or dig or they could get hurt by the landmines. There are special people who can find the landmines and remove them safely so people can use the land without being afraid of stepping on them.
Landmine contamination in Bosnia and Herzegovina means that lots of parts of their land have invisible toys that can hurt, so people have to be very careful or they can get hurt until the landmines are removed.