Smaller screens and a potentially less precise interface means an increase in mistakes. This new addition makes correcting those mistakes fast and simple.
"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:
This bar stays in position even if you move to another screen (e.g. moving to ‘Menu’ from ‘Inbox’). If you don’t happen to catch your mistake in time, not to worry: all four actions can still be undone through other means (e.g. you can move a message from Trash back into your Inbox)."