This is the 6342nd most frequent German word.
drob
"Drob" is not a standard German word. It may be a typo or a regional/dialect term.
Here, 'drob' expresses the cause or reason for the happiness.
Er freut sich drob, seine Freunde wiederzusehen.
He is happy about seeing his friends again.
The word 'drob' is used to indicate the subject of thought—what the speaker is reflecting on.
Ich denke drob nach zu gehen.
I am thinking about going.
In this sentence, 'drob' denotes the reason or the preceding event leading to the discussion.