Fair question. Foreflight is a mobile application you can run on your tablet or mobile device.
As a software, its job is to act as your Electronic Flight Bag (EFB). An EFB allows you to plan a flight, execute your flight and analyze data post-flight digitally via a mobile device.
Thus, Foreflight replaces paper charts and gives you all the information you need for your flight and situational awareness—including weather, runway, procedure, NOTAMs, and more—right there on your iPad screen. This information overlays on a moving map centered around the current GPS position of your aircraft.
To get the best out of Foreflight, you should combine it with a compatible ADS-B receiver.
The receiver takes live ADS-B weather and traffic information and connects with your iPad through WiFi.
The ADS-B receiver allows Foreflight to overlay real-time traffic and weather information onto your moving map while airborne. Neat.
Please take into account that there are different subscriptions for the Foreflight app. Higher subscriptions allow more significant levels of functionality and real-time information.