Google has recently added a security feature in gmail that is based on Country-Accessing. As per it, if some other person accesses your gmail Id from any other country then google will prompt you a warning message. Gmail servers will check if your gmail account is accessed from 2-3 different geographical locations of the world within some minor intervals of time. And if is accessed, you will beprompted this at your next login when you open your mailbox from your usual geographical location and ip.
The warning message will contain a ‘Details’ link that would be reporting suspicious activity. The link will display the Access Type (Browser, Mobile, and POP3 etc.), Location with IP address and the time and date (which will be displayed in your time zone).
If you think it as someone misusing your gmail id, you can change your password else can dismiss the message if your husband/wife or known friend is using your account.
This is a very useful feature introduced by Google in its mail server as it adds a top level security layer. If any hacker tries to access your account and if he/she is from some other geographical location then at your next login, you know that your id is accessed illegally by someone. You can immediately change your password and let your precious mails and information secure from being leaked.
On the other hand, if your spouse or any known friend is using it then you can ignore the Google Warning Prompt and he/she can continue without any sort of interruption.
You can read more this at gmail blog at Detecting suspicious account activity