Confused on how to "create executable code"

The help of those who learned C++ through C++ Primer Plus 6th Edition would especially be appreciated.

On Page 18 of the book, there is a section that teaches how to create executable code: The Mechanics of Creating a Program.

The first step is simple; it is simply to write your code in a file and save it.
The second step is to compile it.

The third step is what confuses me:

"Link the object code with additional code [what does that mean?]. For example, C++ programs normally use Libraries. A C++ library normally uses your object code with object code from the functions you use and with some standard setup code to produce a runtime version of your program. The file containing this final product is called the executable code."

There's the exact quote from the book.

So I guess my questions are:
1. How do you "link" these pieces of code and what does that mean?
2. What exactly are libraries (I have looked it up, but have not found good understanding?
3. A explanation of that paragraph is not necessary, but would be appreciated.

Thank you!
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