When countries want to work together, sometimes one country will let another country use something that belongs to them. This is called a concession or lease.
A concession is like when your friend lets you borrow their toy for a little while. The country that owns the resource or land is letting another country use it, but they still own it and can take it back whenever they want.
A lease is like when your family rents a vacation house for a week. The country that owns the resource or land is letting another country use it for a set amount of time, but they still own it and will get it back after the lease is over.
These concessions and leases can be for all kinds of things: land for a military base, access to natural resources like oil or water, or even a building for an embassy.
Countries make these agreements because they can benefit both sides. The country that is using the resource or land can gain something valuable, and the country that owns it can receive payment or other benefits in return.
It's important for both countries to follow the rules of the agreement and treat the resource or land with respect. If something goes wrong, the lease or concession can be ended early or even cancelled.