Wordsworth said that poetry is "the spontaneous overflow of emotion reflected in tranquility." He discusses this in more detail in his Preface to Lyrical Ballads. He wrote about passion and the imagination as supplements to scientific knowledge. His insistence on the inherent connection between mind and nature is reflected in his ideas that passion, emotion and the imagination are more direct conduits of that mind-nature connection that, say, scientific analysis or reason. He also wanted his poetry to address the themes and experiences of common people; to really get at these daily experiences, to understand real experience.
In "I wandered lonely as a cloud,” in the third stanza, the speaker says, “I gazed—and gazed—but little though/What wealth the show to me had brought:” After describing these dazzling observations of nature, he says that, at the time he observed them, he gave little thought to their significance. It wasn't until later when he recollected these experiences that this overflow of emotion reflected in tranquility occurred. And it wasn't until this imaginative and emotional reflection that he really understood the significance of those experiences in nature.
So, as a comment on the limits of human knowledge, he is saying that there is an emotional, reflective, mental aspect of thinking that must accompany (or follow) empirical experience in order to fully grasp the significance and profundity of that experience. In other words, to overcome the limits of human knowledge (to get the most out of experience), one must supplement the scientific analysis and empirical observations of the world with imaginative and emotional reflection.