This first digit indicates the major database version and it describes the nature of release. Oracle releases a new release every four years on average.
For example, Oracle 9i (internet), Oracle 10g (grid), Oracle 11g (grid), and Oracle 12c (cloud)
The second digit represents the software’s maintenance release number. Oracle releases the major update as maintenance release 1 and then follows up with a second maintenance release later in the software’s lifecycle. With maintenance releases, new features are introduced to database software.
For example, Oracle 12c release 1 and Oracle 12c release 2
Fusion Middleware Number is the third digit. This will be 0 for database software.
This fourth digit is called component-specific release number, and a component release level is identified by the fourth digit. Depending on component patch sets or temporary releases, different components can have different numbers in this role.
Example:  12.1.0.1; 12.1.0.2; 12.2.1.1
A platform-specific release is identified by the fifth digit. This is usually a patch kit. This number will be the same across all affected platforms when different platforms need the same patch package.
Example: GA release (initial release-no patch): 12.2.0.1.0