A land mine (called a "mine" when a naval mine is clearly not what is meant) is an explosive weapon that is put on the ground or just underneath the ground so that it explodes when a person or vehicle goes by. They are called "mines" because people who put them there often dig a tunnel under the ground (like miners who dig for coal etc.). Many people can be killed by mines, including civilians. Sometimes mines can lie in the ground and kill and injure people years after a war has ended.
There are different types of land mines. Big anti-vehicle mines are against vehicles. Smaller infantry mines are used against foot soldiers.