Something that is acceptable is satisfactory or proper for whatever purpose it is considered; it may not be all that’s hoped for, but it's at least adequate. A grade of “A” is best, but is a grade of "C" acceptable? It depends on whom you ask.
The word acceptable means "able to accept," a word that comes from the Latin acceptare, which meant "to take willingly." Although the word acceptable suggests something that is fine, it sometimes suggests the least that would be sufficient, and can carry a somewhat negative connotation, as was the case when Franz Kafka wrote, "Start with what is right rather than what is acceptable."