A smile can convey understanding, joy, or appreciation of humor.

Literal

Smile [topic-は] understanding, joy, or humor [object-を] understanding thing [object-を] conveying [can-ことが出来る].

微笑みは...伝えることが出来る lists what a smile communicates. ユーモアを解したこと means 'having appreciated humor.' A thoughtful observation about nonverbal communication.