A javelin is a kind of spear that is usually used in competitive sport, where the person who throws the javelin the furthest wins.
Landmines are explosive devices that are triggered when someone stands on them or a vehicle drives on them. An area that is studded with landmines is called a minefield. The mines are usually buried just under the ground so that people can't see them. They are used to keep people from going through an area during a war.
The problem with landmines is, after a war is over there may be thousands of landmines still left in the ground, and noone knows exactly where they are. So they kill people when they go to start a farm, or children who are exploring. Even the process of removing them is extremely dangerous as they can explode at the slightest touch.
Military means anything relating to the armed forces, particularly the army.