To email a candidate, open their profile (click their name in the candidates list), then click their email address — shown with an envelope icon and styled as a link.
It opens a new message in your default email app, already addressed to the candidate with a subject line filled in for you (for example, "Your application to [Company] - [Job Title]").
You can also email someone straight from the candidates list. On a wide screen, each candidate's address is shown in the Email column: hover over it and click Send Email on the small card that appears. Clicking the address itself copies it to your clipboard rather than opening your email app.
On a small screen or in a narrow browser window, the Email column is hidden. Open the ⋯ (more actions) menu on the candidate's row instead and click their email address there.
If you don't have a default email app set up, copy the address and paste it into whichever email tool you use. You can also just reply to the candidate's application email — your reply goes directly to them.