What does Address Reuse mean?

It is an unintended practice, reducing the privacy and security of the participants of the transactions.

Most wallets use deterministic keychains and mask the generation of new addresses from the user making it easy and safe to maintain privacy.

Problems with Privacy

Address reuse harms the privacy of not only yourself, but also others - including many not related to the transaction.

Every time a re-used address's private key signs a fresh transaction, the receiving party can use the history of that address to discover information about that address and its owner.

The relationship graph in a re-used address is powerfully-linked in that all of the inputs to that address are necessarily joined (via the spending authority of the private key) to all of its outputs.