A newbie is someone to which a certain subject is new, hence the person would not yet know how to behave correctly or what can and can't be done. For example, if someone has never played Tremulous, then downloads/installs the 1.1.0 client and joins a random server, this person would not know how to properly build a base, that the current weapon has to be sold before a new one can be bought, and the person would probably overall be very much standing around, trying to figure out controls etc.
There is nothing wrong with being a newbie, even though Internet folk tend to toss the term "noob" (which is an established slang form of "newbie") around on every possible occasion and oftentimes in a derogatory fashion. But hey, it's the Internet. Everyone's an asshole just because we can. Sadly. scnr
Another slang form of "newbie" is "nub", but its meaning might be slightly different. Seriously, this is getting ridiculous.