Spooks use the same tools to catch whistleblowers that they use to find spies. They monitor content and metadata of communications. Then when there's a leak, they try to backtrack by analyzing metadata.
For experts it's been possible for many years to protect the content of email. GPG works. Snowden relied on it. GoodCrypto made that content protection available to ordinary people, even children. But almost no one protected the metadata.
The metadata is all the information on the outside of an envelope. Who the mail is from, who it's to, when it was sent, etc. That information has been used to catch even very careful whistleblowers.
GoodCrypto now protects your metadata. Not just with encryption, but with packetization and padding. That means no one listening on the line can read or analyze any individual's metadata.
If you're a whistleblower you use any mail service that supports GoodCrypto. The example Snowden gives is a whistleblower sending mail to the Associated Press. All that shows is that the mail service and the AP communicate sometimes. Exactly who, when, and how much are all invisible to anyone spying on the line.