Plagiarism is the passing off of another's work as your own.
For example, copying and pasting from something somebody else has written without giving them credit (regardless of the license they have used) is plagiarism. This means even text that is in the public domain can be plagiarised, because even though the authors' might not mind it still appears as if one has stolen their words.
Plagiarism is against the rules and will be dealt with accordingly and on a case-by-case basis.
If you need to use others' words in a quote, familiarise yourself with the appropriate templates and make sure you know how to reference. If you are worried use the sandbox namespace (which is private) instead and wait for a sysop to have a look.