Definition
The term "lowbie" is essentially a shortened version of "low level," meaning content designed for players who haven't yet progressed very much through the game. What counts as lowbie content varies wildly based on the opinion of the player speaking, although in general any type of content intended for characters below level 60 can be labeled as such, whether it is dungeons, battlegrounds or quests. In some cases, raid dungeons designed for characters below the current level cap can be considered "lowbie raids" regardless of the level they were intended for or the expansion they were released in. For example, you may encounter players calling the Trial of the Crusader raid dungeon a lowbie raid, despite the fact that it was designed for level 80 characters.
Usage
You can use the term "lowbie" both as a noun and as an adjective, maintaining the same meaning. For example, you could say "I spent all of yesterday clearing lowbie dungeons for the achievements" or "I was just standing around in Orgrimmar minding my own business and a lowbie kept whispering me begging for gold." The plural form of "lowbie" is "lowbies"; you may occasionally encounter players who shorten the term even further to "lowb" or spell it "lowbee" instead.
Lowbie Versus Newbie
Although the term "newbie" is very similar to "lowbie," these two terms do not have the same meaning. "Newbie" refers to players who are new to the game, regardless of their current level. Sometimes these two terms coincide, since every character in the game counts as a lowbie when tit is first created. However, while exploring the game, you may encounter lowbie characters controlled by experienced players. Conversely, there may also be characters at the maximum attainable level controlled by newbie players who are still relatively unfamiliar with the game.
Considerations
Game content intended for characters below level 60 used to be quite outdated due to remaining in the same state since "World of Warcraft" was launched in 2004. However, the content was entirely redesigned in the "Cataclysm" expansion to take advantage of all-new game mechanics and ensure that each low-level area has an engaging storyline and provides useful rewards. Although some players may not enjoy participating in lowbie content and will speak about it in disparaging terms, this simply reflects their personal opinion rather than an objective fact about the game or the game environment.