This quote comes from "The Boscombe Valley Mystery," in which James McCarthy is accused of murdering his father Charles. The circumstantial evidence points to it being "obvious" that James killed Charles: James walked into the woods with a gun and quarreled with his father. A short time later, Charles was killed with a gunshot. To Watson, the case seems open and shut, but as Holmes points out to Watson, "obvious facts" can be deceptive, and sometimes to see this, you need to "shift your own point of view a little, [and] you may find it pointing in an equally uncompromising manner to something entirely different."
One TOK, or theory of knowledge, is sense perception. This is the basis of empiricism or the scientific (experimental) method. Direct observation has been important to the advancement of knowledge. We find out, for example, that the world is not flat by sailing around it and observing that the ship never falls off the edge.
However, as Holmes would be the first to point out, sense perception, too, is subject to revision through shifts in perspective. For example, even some of Galileo's closest, most highly educated friends were aghast that he argued that the earth revolved around the sun, because their senses clearly showed them the sun moving across the sky. Galileo, however, had a different point of view: his minute studies of moons and planets through his telescope allowed him access to a fuller and truer picture.
Another TOK is reason. Watson uses reason when he determines it is obvious from the chain of events that occurred that James went into the forest with a gun, quarreled with his father, then shot and killed him. But as Holmes understands, just because one thing happens after another doesn't mean the first thing caused the second. Reason must be tempered by a clear understanding of logical fallacies, such as false cause. Jumping to conclusions without studying the gaps in the evidence can lead to error. For example, Einstein's ability to reconceptualize or shift his perspective on the relationship between different aspects of the universe, such as time and gravity, revolutionized physics.