“As the doctors say, when you hear hoofbeats, you don’t think zebras."
This means that when people perceive something (that does not directly tell them all the relevant information), the first thing that comes to mind is something more common or something commonly thought of. For example, when people see smoke, they usually immediately think "oh no there's a fire!" instead of thinking "ooh, it's a campfire" or "ooh, a million people are gathered over there, smoking away" (hahaha).
"Also, folks have a tendency to see what they’re looking for, don’t they?"
This means that people are more inclined to see something that they have been thinking about a lot, or something that they had already thought was there. (They might or might not be fully conscious that they had these pre-existing beliefs.) This is called a confirmation bias. Before Dr. Haskell saw the x-rays, he had probably already thought that Sam was suffering from a serious illness. So when Dr. Haskell finally saw the x-rays, there were probably some irregularities in them (like some abnormal dark spots or something) and Dr. Haskell thought "well it's not as serious as cancer, but it really is something else serious like I had previously thought!"
[SPOILER ALERT: The following contains possible spoilers for the TV series, House!] (sorry, had to include this warning just in case people reading this intend to watch House haha!)
What is the relation between “died a hero’s death” and “Gregory House”?
Gregory House is a fictional character (a doctor) from the American TV series "House". I don't watch the series but I understand from a quick glance at the plot summary that Gregory House fakes his death in the finale. But I think I need more context in order to understand the comparison made between Haskell and House. How did Dr. Haskell die?