You can see the estimates statuses on the Estimates grid:

Estimates can have six statuses:
1. Draft.
Status appears if you click “Preview” after estimate creation:

2. Activated.

Status appears if you click “Activate and Send” after estimate creation…

OR if you send to your client the “Draft” estimate or you “mark as activated”.

3. Viewed – if the client has opened (viewed) received the estimate.
4. Replied – if the client has replied (commented) on the estimate (more here).
5. Accepted – if the client has approved the estimate…

Or if you did it by yourself.

6. Invoiced – if you have generated an invoice based on this estimate.

Note: If an Estimate is already Invoiced then it can’t be edited anymore (neither changed back to activated nor approved).