The title tells us that this poem is an anecdote, or a little story, about a jar. When we read this title alone, we can think about a jar, and all the things that a jar can do. It can store food for consumption later, it can hold fun little buttons, it can capture a bug that we've caught. In the verb sense of the word "jar," it can shock us, suddenly, changing our routine and everything that's normal.
When we think about an anecdote, we think about little stories, sometimes they are funny, sometimes they are moral lessons, and sometimes they're just stories. But when we read this poem, it doesn't really seem like a story at all to us. It's just talking about a jar. The only plot to the tale is that it's taking over the wilderness.
Yet, it's also a kind of relief that the tale of the jar taking over all this wilderness and making it no longer wild is just an anecdote. If this were truly, literally happening, this would be a disaster of epic proportions, not just a nice little story. So, we've got to think about the irony, the unexpected meaning, of the title as it relates to the rest of the poem. It's more of a warning, it seems like, than a description of actual reality (although, you could argue that humanity has, indeed taken over the wilderness—we'll listen to you!).
The title is also important in one possible interpretation of the poem: to read it as in discussion with a famous poem by John Keats called "Ode on a Grecian Urn." The urn is beautiful in itself and decorated with scenes of lust and joy and life. The jar is this fat round little thing, sitting on a hill. It's not ancient, Grecian, or beautiful at all. It just is. Yet the jar—like the Grecian urn—as a mere object has some sort of strange power over the growing, living world, at least in the eye of a human beholder.
So, maybe, especially through the echo in the title, Stevens is saying that he, like Keats, can see the power of objects to say something about life. In the hands of an artist like Stevens, a jar is more than a crummy, little jar. It's the catalyst for this story that he's telling. It's a vessel, filled with ideas about our role in the natural world.