A blockchain bridge lets an asset native to one network, Bitcoin for example, be represented and used on a different network, such as Ethereum. It does this by locking the original asset on its home network and creating a corresponding token on the destination network that represents it.
Two ledgers, one movement of value
From an investigative standpoint, a single bridge transaction creates two separate records on two separate ledgers describing the same underlying movement of value. Reading only one side of that record gives an incomplete, and sometimes misleading, picture of what actually happened.
A legitimate tool that also adds friction to a trace
Bridges are used constantly for entirely ordinary reasons, moving liquidity, accessing a specific application, or simplifying a portfolio. They are also a common step when someone is deliberately trying to obscure the origin of funds, since each additional network requires its own tooling to follow correctly.
Reputable bridge services keep transparent, publicly available transaction records, which is exactly what makes it possible to reconnect both sides of a movement even when a bridge is used deliberately to slow a trace down.