To drop by is to make a short casual visit without arranging it. Drop by for a cup of tea, drop by later, feel free to drop by.
The lack of arrangement is the whole point, which is what separates it from calling round by appointment. It is friendly and informal, and it implies you will not stay long.
1phrasedrop byTo make a short, casual visit without arranging it.
“Drop by any evening this week and see the garden.”
drop by is a phrase.
Also accepted in games: drop in, stop by
Practised in Phrasal Verbs