When we say copyrighted, what does it really implement or mean?

Everything a person creates is copyrighted material and belongs to the original creator, provided it is not from another source, however open source is put under the GPL license agreement for people to use the original source code, change, modify and alter it in some way or another and to provide their version for other people to do the same thing, without worry of copyright violations. Some open source software, may involve other restrictions in what a person can do. Linux is a prime example of the open source files and software. You really need to read what the license agreement is.