What does it mean to sweep or redeem "dust" amounts of BTC?

For all Counterparty transactions there are so called redeemable fees (for more on that see here) and their exact number depends on the type of transaction. For each transaction there are 2 or more multisig outputs and there can be more. BTC_Pay (see link above) needs three multisig outputs.

This means that an address with many transactions can spend a non-insignificant amount of “dust” (very tiny) amounts of bitcoin which optionally can be redeemed. The amount must be enough to cover one transaction fee, so if there’s less (balance of any bitcoin address can be checked on blockchain.info), send the missing amount so that the total is 0.0002.

When Counterparty implements 1-of-3 mutlisig these fees will be lower (more data in each outputs means less outputs), but currently 100 transactions can generate approximately 0.000156 BTC in redeemable fees which can be reclaimed.

There are 3rd party tools that can help users to sweep the blockchain and reclaim these fees and send them to the owning address. A more private and reliable way is to use a trusted wallet (e.g. Electrum or other). Another is to use online tools. One of them is given below. (Counterparty does not specifically endorse or recommend any specific tool.)