Could some please explain to me what account sharing is exactly?

Like is account sharing just mean two people on one account or two people who know each other using two accounts and sharing items and land?

Account sharing is anyone using an account other than the account creator.

For some reason, a lot of people consider this to be "no big deal". However, your account is your online identity. Letting someone else use it makes you responsible for anything they do as if you did it yourself. You deal with whatever consequences result from it.

It's much like lending your car AND your drivers license to a friend. If they crash your car, you are the one with a broken car. And you are also likely to be in trouble for false identity for lending your license and allowing your friend to claim to be you.