These are highly simplified explanations:
Bitcoin is a cryptocurrency - think of it as a virtual commodity that has no real physical presence - the value of one unit of bitcoin varies.
Now real commodities - say diamonds or orange juice or beef - these are farmed or mined by someone - who sells it to someone who then trades it en masse.
Bitcoin works like that. Someone "mines" bitcoin into existence - after that they own it and can sell it on.
The way you mine bitcoin is as follows:
Cryptocurrency is underpinned by an extremely complex maths problem - for arguments sake let's say something like factoring and calculating prime numbers. Its easy to do the first few, but after that it becomes very very difficult. Let's say the bitcoin mining problem is a maths problem that has a number of solutions but involve absolutely enormous numbers. Everytime you solve the problem with a new solution - you bring a new bitcoin into existence. This work is the equivalent of someone digging the ground to find gold or diamonds, or spending the time farming cattle or growing lemons to sell.
The thing is - the longer it goes on, the more people mine bitcoin, the more unimaginably huge the numbers get. The only way to mine new bitcoin is to use incredible computing power. We are at the stage now where people have crypto "mines" in countries like Iceland with enormous banks of supercomputers working round the clock to mine new coins. The cost of this is making it almost rather marginal, such is the expense of tech and electricity - and they're doing it in cold countries to save on cooling.