。
Most things have a front and a back.
Literal
Things [to-には] mostly front [and-と] back [subject-が] exist.
ものにはたいてい ('most things have') is a natural generalization pattern. 表と裏 ('front and back, surface and hidden side') is both literal and metaphorical — the saying implies nothing is purely what it appears. This reads like the opening of a philosophical observation or life lesson.