A truism is a claim that is so obvious or self-evident as to be hardly worth mentioning, except as a reminder or as a rhetorical or literary device.

I really enjoy the style and rhetoric found in Southern literature. Mark Twain, William Faulkner and Erskine Caldwell were perhaps the best practitioners of this art

This truism you mentioned is somewhat similiar to one attributed to the Greek philosopher, Aristotle – “One swallow does not a summer make”.

I think of it as a colorful way for people to say “Let’s not get ahead of ourselves here and jump to conclusions”

It means there are limits. It can also be used to get people to accept a lack of effort or laziness by justifying it with an obvious “word of wisdom”.