"The Lake Isle of Innisfree" is the name of a place. It's a pretty straightforward title in a lot of ways—it sets us up to hear about the place and to find out what relationship the speaker might have to the place. All of those expectations are fulfilled when we read the poem. So in that sense, it acts as a hint of what's to come. Simple enough.
But the title, while informative, also has a bit of a poetic flourish. In real life, the Lake Isle of Innisfree is usually just called Innisfree, though it is an island (an isle). It's kind of like calling Manhattan "the River Isle of Manhattan." The fanciful title prepares us for the dreamy, picturesque place that the speaker describes as Innisfree. It's not just any place. It's a grand place—the stuff of daydreams.