Sometimes when you are using Gmail on your smart phone, you might delete a message by mistake or label it incorrectly. Special with touch options there is a great possibility to touch the wrong button :). Sure you can fish the message out of your Trash or remove the label and apply the correct one, but that takes several steps. Even just a few seconds is usually enough time to catch those annoying mistakes.
Now when you use the Gmail mobile web app, you’ll have a small window of opportunity to undo four key actions: archive, delete, add or remove a label, or move a message/conversation.
When you take one of these actions, Gmail displays a yellow bar that recaps what you just did and allows you to undo it:
Undo certain Gmail actions in your mobile browser - Official Gmail Blog