Real time apply means that the standby database is applying the redo -information as soon as it hits the standby redo logs. If you're not using real-time apply the redo will be applied after a logswitch has occured on the standby database.
A 'normal' (non Active Dataguard) standby database is in mount mode, and is not open for 'user-queries'. In the normal case, the standby database can be opened read-only, but then managed recovery have to be turned off (meaning it is not applying redo anymore). In an active dataguard configuration the standby database is open read only but at the same time applying redo as it is sent from the primary database. So in that case the standby database can be used for offloading reporting (as an example) using up-to-date information.
Active dataguard is a separately licensed option.
In addition to the valid answer: Active Data Guard is an option that includes several features.
The most famous is Real-Time Query, which was referred to by /M.
But there is also the feature Block Change Tracking on the Physical Standby.
In 12c, Active Data Guard got even more enhanced and includes there also Far Sync Instances and Real-Time Cascading Standby.