Okay, so we've got three words in the title, but only two of them that really matter: "solitary" and "reaper." No, the reaper that our speaker has in mind isn't this guy or these guys, but rather something more like this. So the title tells us that the poem is going to be about somebody who gathers crops, and that somebody is all alone—solitary, with nobody else around (except the speaker, that is).
Now, there's a little bit more to this title than you think. If you were alive in 1807, and had read a bunch of Wordsworth's poetry before that date, the title "The Solitary Reaper" would probably have reminded you of a few other things. First, you might say to yourself, "Oh Wordsworth is writing about farmers again. What surprising and interesting things are they up this time?" That is one of Wordsworth's trademarks: farmers and peasants doing slightly strange, but also very thought-provoking things (trading old wives tales, sharing famous legends, singing traditional songs, that sort of stuff).
Second, you would probably also say to yourself, "Hmm, this is probably another poem about solitary artists out in the countryside." Wordsworth's poetry constantly explores the Romantic obsession with the solitary genius. The works of Wordsworth and the other major Romantic poets (Blake, Shelley, Byron, Keats, and Coleridge) all feature extremely gifted, but troubled "loners." Everybody who was into the whole poetry thing in 1807 would immediately interpret this poem's title as another installment in that series (don't forget that the highland woman is, essentially, a poet).
Want to know more about this solitary genius theme? You can read about it here if you like.