Not OP but I figure I'll pitch in because nobody else explained it. When referring to backed vs unbacked Bitcoin (or any other UTxO accounting model based cryptocurrency), the "backing" isn't about whether the token is backed by another asset but rather whether your proxy holding of the token is backed by the actual cryptocurrency on the ledger.
If it's some wrapped Bitcoin that is backed, that means that there exists some UTxO which corresponds to the wrapped bitcoin tokens you hold. UTxO are atomic and unique so you can verify that your wrapped tokens have a 1:1 mapping to the tokens held within the UTxO on the native token's network. Non-backed bitcoin instead are just tokens that are handled through some minting mechanism where there is claimed to be a 1:1 equivalence without any actual tooling to prove that.
This concept of backing can actually even be further extended via Secure Multi-Party Computation where private keys can be collectively held in such a way that no user ever can see the private key but together they can coordinate to build and sign transactions with that private key without leaking it even to themselves. This can then be used to actually hold tokens from a foreign/incompatible network and interact with that network with little to any required trust.
Hopefully that helps answer what is meant.
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Also since it's often a question asked I'll answer preemptively: "What is UTxO?"
I've mentioned the UTxO accounting model a few times here. Bitcoin doesn't have accounts but rather UTxO or Unspent(U) Transaction(Tx) Outputs(O). These are indivisible bundles of tokens that can only be created or consumed/destroyed atomically. There are no accounts but rather just private/public key pairs which can redeem/spend those UTxO. Additionally, all transactions are just a collection of input UTxO (that are consumed with the tx) and newly created UTxO. All Tx must uphold the rule that the sum of all inputs equal the sum of all outputs (or some equivalent of this rule as is the case for Monero's Zero Knowledge transaction proofs). A number of networks use UTxO or a UTxO extension (such as EUTxO or the cell model). Most famous is Bitcoin but Monero, Litecoin, Cardano, Ergo, Nervos, and a number of other networks also use this model.
This is opposed to Ethereum and co (Polkadot, Solana, Luna...) who use the balance-account accounting model. This model works like traditional ledgers where tokens are stored as some balance in an account and transactions are just transfers from one account to another.
