The Protocol Fee Sweeper is a helper contract to convert protocol fees collected in a wide variety of tokens to a common "target" token for the treasury (e.g., USDC).
It does this for each pool by withdrawing tokens from the Protocol Fee Controller to itself, and calling a configurable "burner" contract (e.g., from CowSwap) that does the actual conversion swaps, and forwards the proceeds to a recipient (e.g., the treasury).