It means copying the contents from another websites.
The process of rewriting the contents from another website is known as Patchwriting.
If the contents of any websites are similar then the Google identifies the website which have copied the contents and lowers its rank in the search engine results page.
Plagiarism is difficult to detect unless you have read information from original source.
CUT AND PASTE
It is another method of copying the information from one website.
It is natural that some words are same when many writers are writing about a same topic.This is not penalized. But it is not possible that each and every word is same.Still many uses this method.
It is also possible that many writers use same word or same phrase in their text.This is accidental plagiarism.Sometimes a writers copies his own work without knowing.
PLAGIARISM GOES BOTH WAY
Plagiarism goes both way.You can copy any information from other websites.Similarly,it is possible that someone stoles information from your website.
Sometimes your whole website is stolen.In that case,google considers you as the problem and blocks your website.
One should know what to do if information from his website is copied.
TOOLS FOR AVOIDING PLAGIARISM
It is possible to avoid plagiarising other websites.Check your contents before and after posting it by using some tools.One such tool is Copyscape.It is low-cost tool and provides better results than other tools.
Another tool to do so is IFTTT which stands for “IF THIS THEN THAT”.This tool creates e-mail alerts when your information is shown on other websites.
To avoid plagiarism,you should understand copyright laws.
One should avoid using this method as this will create problems for other websites that he will create in future.