If you are reading about pen & paper roleplaying games on various blogs and forums, you sometimes come across the expression RAW, which means "rules as written". Unlike a computer RPG, where most rules are hard-coded into the game and allow no interpretation at all, rules in a pen & paper game are far more flexible. You can play them "as written", or you can modify them if you feel the rules are contrary to common sense or you think the intended outcome of a rule is different from the literal interpretation. For example if your players fight a gelatinous cube and use a power that would trip an enemy and make him fall prone, do you apply that rule as written, or do you declare that the cube is immune to falling prone, because that makes more sense? There is no right or wrong answer to that, and the decision might well depend on the style of your game and the ruleset you use.

Now in my 4E campaign I came upon a different problem with rules as written: Wizards of the Coast sometimes issues errata to the printed rules. If you use the D&D Character Builder on their website, it uses the "rules as written including errata", but to the player the errata are very hard to find (especially now that 4E isn't the current edition any more), and basically invisible. And when those errata are basically a nerf to a character class, that can come as an unwelcome surprise to the player if the DM cites some new rule the player wasn't aware of.

One of my players is playing an Avenger. The Avenger starts out with cloth armor, and a special ability called Armor of Faith that gives him +3 to AC. He can take a talent that improves that bonus to +4, so even in cloth armor his armor class is already quite high for a melee dps. Now the rule as written for Armor of Faith is that this bonus applies as long as you don't wear heavy armor or use a shield. That opens up the possibility to take a talent that allows the Avenger to wear leather armor, and gain another +2 AC. But that turned out to be overpowered, so the errata "fixes" Armor of Faith to give the bonus only if no armor or cloth armor is worn.

The player in my campaign didn't know that and wanted to learn to wear leather armor. I noticed the problem when I tried to make the character sheet in the D&D Character Builder and the software used the errata'ed rules and didn't give the Armor of Faith bonus. Now I could have overruled the errata and house-ruled that in this case "rules as printed" apply. But then the Avenger would have ended up with the same armor class as the two tanks in the group, while dealing significantly more melee damage. So I went with the errata and told the player to forget about the leather armor and take a different talent. Ultimately my decision was based more on what would be more fun and balanced, than on legal niceties.