How to check oracle error description in sqlplus?
You can use oerr utility.

Example:

[oracle@ora12c Desktop]$ oerr ora 01950
01950, 00000, "no privileges on tablespace '%s'"
Cause:    User does not have privileges to allocate an extent in the
          specified tablespace.

Action:   Grant the user the appropriate system privileges or grant the user
          space resource on the tablespace.


You can search error code on this webpage Search for Error Messages or you can download the Database Error Messages documentation from here.

Note: This utility is not available on Windows.