It is usually very easy to tell what a given tone poem is about - becase in an insane act of clarity, composers usually named the tone poem after the thing it was written about. Well, I'm sure the painting was of a Hun, or Hun-like battle. So, applying the assosiative property, the symphonic poem is about Huns. It is programmatic. Programmatic music is music that has its roots in the real world.
Program music is written to sound like something, or give the impression of something.
Like, there may be a light flute passage to signify birds, or a drum interlude meant to sound like clockwork, or droning trombones to sound like B-17s. You can also get across concepts programmatically, like dying, throwing objects, or fighting. Most music is not program music. Absolute music is sort of music for the sake of music - not correlating directly to anything tangible. Absolute music is that a tone poem is not.