Well sure, technically the Bitcoins are not lost until they get permanently burned. However, if you lost the private keys of your Bitcoins then it is very likely that they will never ever be transferred again. So those Bitcoins are acutally out of the "game".
Can someone please explain me how bitcoin can be lost?
I don't feel they can ever be lost, losing the ability to access your wallet is the worst that can happen, but in reality the coins are still there.
They can be lost forever. For example, you have thousands of bitcoins and it makes you feel unsafe if you will just store it for a single bitcoin wallet. So what you did is you create another wallet for distribution of your some bitcoins and partitioning it. But as the day goes by, you forgot the private key of your account and as well as your email. So basically, the balance bitcoin on that wallet will be lost forever.