The Proxy authentication mechanism allows a user to login as their own user but land into a different schema without knowing the password for that user. For example the USER_B may want to create an object in the SCHEMA_A or want to use the privileges/roles of the SCHEMA_A user.
sqlplus> create user USER_B identified by USER_B default tablespace user_data;
sqlplus> alter user SCHEMA_A grant connect through USER_B;
You can now connect through your own user but land in as the SCHEMA_A user.
sqlplus> conn USER_B[SCHEMA_A]/USER_B
sqlplus> show user
USER is "SCHEMA_A"
The privilege can be revoked by issuing the following.
sqlplus>alter user SCHEMA_A revoke connect through USER_B;