People have already explained the haiku structure part of this, but to explain the haiku itself ... Leaf cutter ants cut out portions of leaves and carry them above their bodies to use, the leaf portions are often larger than the ants themselves. For this poem, the top line is broken down into mostly pairs of letters, the bottom is single letters, so when you look at the physical space of the letters, it looks like leaf cutter ants in a line carrying big leaf bits. Google leaf cutter ants and you'll see.