Definition - Geo-targeting is an optimization technique used in marketing to target users based on their location. This is usually captured by IP address, location based services (mobile), country, state, city and other means.

Marketing - The advantage to using geo-location for marketing is narrowing down marketing so that it reaches a targeted audience based on that locations specific demographic and has the highest conversions.

Optimization-

Whenever optimizing a given marketing channel - geolocation is a critical way to maximize ROI and conversion rates. It’s recommended to Increase the CPC/CPA/CPL for high performing locations.
In combination with utilizing day-parting, geo-targeting is the most common optimization technique.
The latest common technique for geo-targeting is a specific technique called geo-fencing. It’s the use of geo-targeting users when they get into a specific area commonly used in social media (facebook, yelp, etc).