Something inexact is vague or not quite correct. If a doctor gives you an inexact diagnosis, you might want to see a specialist for a more specific opinion.
When a poem's translation from one language to another is inexact, a lot of the original meaning can be lost. Some things, like weather prediction, simply have to be inexact — you could call meteorology, or the study of weather patterns, an "inexact science." The Latin root is exactus, "precise or accurate," with the added prefix in-, or "not."