Reply later and notes
Select Reply later above a conversation to keep it on your own list until you reply. It is in the sidebar, and on Overview under Waiting on you. When something has waited 3 days or more, Mira reminds you. See Reply later.
Notes
Select Add note to keep a few words for yourself on a conversation, like "Agreed €4,000, invoice after launch". Only you see it. See Add notes to conversations.