Per MOS:LARGENUM (and various guidelines and projects), most large numbers reported to unreasonable precision should be rounded. While a source might say a film grossed $75,763,102, this figure is unlikely to be accurate. Surely there were some unrecorded cash ticket sales, sales attributed to the wrong film, etc. Additionally, the figure is unwieldy; people generally want to know if it was $75 million, $20 million or $250 million. For Wikipedia's audience, the difference between $75,763,102 and $75,764,063 is trivial.
Rounding creates a more convenient -- though less accurate -- number. So, for example, Wikipedia reports that Star Wars grossed $503.1 million though the source says $503,075,849.
"Rounding" in everyday usage can reflect a number of differing practices. How much were those new shoes? The shelf price was $148.95. If you said "$148.95", you were being exact. If you said "$150", you were rounding. If you said "$148", you were truncating. Heck, some people might say "$140", which is really rough.
Truncating is simply dropping the digits beyond the point you are reporting. It is simple, but far less accurate. Rounding is a bit more difficult, but more accurate: $148.95 is, after all, much closer to $150 than $149. $140 is getting further away.
Basically, that's what rounding is: Saying which of two numbers the "real" number is closest to. To the nearest $100, $148.95 is roughly $100 (it isn't closer to $200). To the nearest $10, $148.95 is $150. To the nearest dollar, $148.95 is $149.
To the nearest $100,000, Star Wars $503,075,849 is $503.1 million.