At first glance, it would seem that the title is fairly self-evident. Here's a kid, dancing with his father. But the title immensely helps out our speaker. It sets the scene of the poem before we even read the first line, letting the speaker concentrate on the rhythm of the language rather than using up precious lines to explain what's going on.
Speaking of the rhythm (see "Form and Meter" for more information) a waltz is a three-beat dance, and each line in this poem has three beats too. So the title fits the form of the poem.
The title also broadens the interpretation of the poem. The waltz could signify the boy's entire relationship with his father, which dances between love and fear.
Some people think the poem suggests that the father is waltzing with whiskey, and alcoholism. We think, though, that the other main partner in this waltz is not whiskey, but the inevitability of death, to which the boy probably loses his father.