Hello Emily,
An agent can handle multiple interactions at once, based on the utilization rules you set in PureCloud. An email is "on hold" if an agent is on a call with another customer, is typing a reply to a different email or is replying to a chat. Same for chat. We only want to count the time the agent is actually spending on the specific email and avoid double-counting the time, so if an agent is actively working on another interaction then the email, chat or call will be on hold.
If you take a look at this article and look at the left hand side, the interaction that has the dark color is the "focus" interaction that they agent is currently working on. All the other interactions are effectively "on hold."
https://help.mypurecloud.com/articles/interaction-page-navigation/
Let me know if this helps or if you have any other questions.
Thanks,
Lucie