1

In Gmail, I send a email then I receive a pop up at left bottom corner:

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What is the use of "Undo" option in this pop up?

2

From a Google answers question:

Recall an email with Undo Send

If you decide you don't want to send an email, you have a short time after to cancel it. Right after you send a message, you can retract it.

and

Choose an amount of time to recall a message

On your computer, go to Gmail.
In the top right, click Settings and again Settings.
Next to "Undo Send," select a Send cancellation period of 5, 10, 20, or 30 seconds.

Google keeps the mails on its servers for the time you specify here. That's how they can actually implement this: they don't immediately send your mail. The default (and minimum) time is 5 seconds.
The mail does not get deleted, you can e.g. edit it and resend.

Note: there are people claiming that in the case of sending mail to another Gmail account, the message is send immediately, because Google "can remove the message from the recipient's inbox."
I have not yet seen proof of that, and I doubt it is true, because it would raise privacy issues.

0

General meaning of Undo is cancel or reverse the effects or results of (a previous action or measure).

In computing it means, a feature of a computer program that allows a user to cancel or reverse the last one or more commands executed.

So, use of Undo is same in Gmail. Gmail add this feature where user can recall their send message in few second by clicking Undo. From Gmail Help:

Recall an email with Undo Send
If you decide you don't want to send an email, you have a short time after to cancel it. Right after you send a message, you can retract it:

  1. In the bottom left, you'll see "Message sent" and the option to "Undo" or "View message".
  2. Click Undo.

Minimum (and by default) time limit of Undo Send is 5 seconds and maximum is 30 seconds. You can change this in Gmail settings:

Choose an amount of time to recall a message

  1. On your computer, go to Gmail.
  2. In the top right, click Settings Settings(gear icon) and then Settings.
  3. Next to "Undo Send," select a Send cancellation period of 5, 10, 20, or 30 seconds.

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