You've planned a route and produced a route file. Now you need to upload it to a GPS enabled device and follow it.
Offline mapping means you don't rely on Internet access as you cycle.
Some apps also let you plan routes, but it's easier to plan on a desktop system and then import a .gpx route file to follow on the phone.
Apps I use the most - each has a page giving further details
Other Apps with offline maps
Most of these apps also track your ride and some can do the tracking part offline:
Strava Tracker (Android iOS) - offline tracking. Upload to Strava or Garmin Connect.
Endomondo (Android iOS) - offline tracking. Upload to www.endomondo.com or Garmin Connect.
Bike Computer (Android iOS) Cycle computer display - synchs with Strava.
Google Maps (Android iOS) Can be used an online bike satnav. No track recording. You can save maps for use offline, but can't get bike directions offline.
Many people swear by (or at) their Garmin, Satmap, Casio unit or watch.