PrivateSend is a feature that prevents balances and transaction history from being known. It uses obfuscation and uncertainty, instead of cryptography.
Why you need a coin with privacy.
Imagine this scenario: you are buying a cup of coffee for $1 and use a cryptocurrency without privacy to pay for it — Bitcoin, Bitcoin Cash, Litecoin, Ethereum, etc. As soon as the transaction completes, the merchant knows he received $1 from address your address XYZ. He goes into any block explorer, checks the balance of the address, sees that you have over $1000, and you followed as soon as you leave.
How Dash solves privacy
Some cryptocurrencies use encryption for privacy, but Dash uses doubt and uncertainty. Coins that rely on encryption get weaker over time because eventually all encryption gets broken, and once that happens, the entire blockchain is no longer private.
Dash’s PrivateSend works by securely “mixing” the funds you allocate to your PrivateSend account with someone else’s so that there is no way to know where they came from. “Bitcoin mixers” achieve similar results; however they are centralized services, and you have to trust whoever is running it not to steal your funds. On the other hand, Dash’s privacy is built-in into the protocol — it is secure and decentralized.