Bitcoin is an immutable ledger of proof-of-work and transactions trading any fraction of the rewards from the said proof-of-work, which is completely trustless and is instead secured by cryptography.

What does this mean?

It's basically a permanent list of "rewards for solving a really hard math problem", and transactions trading these rewards. And it's secure because math guarantees no one can create it from thin air.

It's value is whatever value people choose to give it (dictated by the prices it's traded at in exchanges). And people apparently choose to value it now at around US$7000 each.