When describing two events that occurred in the past but at different times, one uses the Pluperfect tense:

                                 After I had eaten dinner, I watched television.

In English, this tense is formed by using a past tense verb (such as "had") and a past participle ("eaten").
The same is true in German.

                             Nachdem ich zu Abend gegessen hatte  ,  sah ich fern.
                                           PLUSQUAMPERFEKT   SIMPLE PAST
 
 

In German the Plusquamperfekt [Pluperfect] tense is formed by taking the Auxiliary Verb in its Simple Past form and adding the Past Participle.

                                            INFINITIV                             PERFEKT                                PLUSQUAMPERFEKT
                                                essen                              (ich) habe gegessen                                 hatte gegessen

to eat  (I) ate     (I) had eaten

The Conjunction "NACHDEM" (after)  introduces a clause in the Plusquamperfekt.  The verb of the main clause is usually in the Simple Past tense.

Again, NACHDEM introduces a Dependent Clause, meaning the verb appears at the end of the Clause.