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: open the ⋯ (more actions) menu on their row 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.