Real-time overview lets you monitor activity on your site or app as it happens. Each session is reported seconds after it happens on your website. Some useful things you can do with real-time are: view who’s on your site right now, view what city/country the users viewing your site are from, what sources or medium the users came from, what page they are currently on, what device they use and more.
You can access the real-time overview by clicking on real-time at the left side bar of Google Analytics and clicking “Overview”. There are several other real-time reports available that you can access such as: Locations, Traffic Sources, Content, Events and Conversions.