There is more to a bitcoin wallet than just the address itself. It also contains the public and private key for each of your bitcoin addresses. A private key is lost private key bitcoin mathematically related to the bitcoin wallet address, but is impossible to reverse engineer thanks to a strong encryption code base.
If you don’t back up your private key and you lose it, you can no longer access your bitcoin wallet to spend funds. As mentioned, there is also a public key. This causes some confusion, as some people assume that a bitcoin wallet address and the public key are the same. That is not the case, but they are mathematically related. A bitcoin wallet address is a hashed version of your public key. The public key is used to ensure you are the owner of an address that can receive funds.