Both of these expressions mean to pay someone or something an informal visit.

这两个短语都表示非正式拜访某人或某事。

'Drop by' means: to pay an informal visit to a person or a place.

Drop by：非正式访问某人或某地。

It doesn't, however, refer to an informal visit.

但是，它不是指非正式访问。

It's also a way of saying making an informal visit, something that wasn't planned, wasn't scheduled.

这也是进行非正式访问的一种说法，即未计划、未安排的事情。

I'm going to go and see her not for a long meeting probably, certainly not for anything formal, but more as an informal visit.

我去见她可能不是为了长时间的会面， 当然不是为了任何正式的事情， 而是作为一次非正式的访问。

" To drop in" is a two-word phrasal verb meaning to come for a short, informal visit without letting the person know ahead of time.

” “To drop in” 是一个双词短语动词， 意思是在不事先让对方知道的情况下进行一次短暂的、非正式的访问。