Cumulative Updates are essentially hotfixes released by Microsoft for Dynamics NAV. They are commonly abbreviated as CU, and denoted as NAV 2017 CU1, NAV 2017 CU2, etc.
Prior to NAV 2013, Microsoft would periodically release Platform Hotfixes (patches to NAV binaries) and Application Hotfixes (updated versions of NAV objects). Hotfixes were not always cumulative.
Since NAV 2013, Microsoft has taken the route of releasing monthly hotfixes that are cumulative, and inclusive of both the Platform and Application updates. This means that you can skip one (or more) CUs releases, and get fully caught up when you eventually apply the latest CU.
Cumulative Updates for one version of NAV cannot be used with another version of NAV (i.e., CU 6 for NAV 2016 cannot be used to patch or update NAV 2015).
At this time, Microsoft releases monthly CUs for NAV 2013, 2013 R2, 2015, 2016, and 2017. Of late, Microsoft has begun including a zipped copy of the latest build of the NAV installation media (corresponding to the CU build number) with the CU download.
Deploying the Platform portion of the update is simple, and entails the replacement of various files on the NAV Server and Client workstations. Deploying the Application portion of the update requires more effort, and access to a NAV Developer license, and is therefore typically undertaken by NAV resellers. It involves comparison and merging of updated object code into existing objects in the NAV database.