What is ‘mapping’ and how can I use it to my advantage?

The fontspec manual mentions a feaure called mapping. However, it does not really document lots about it:

Mapping enables a XeTeX text-mapping scheme, shown in Example 40.

Using the tex-text mapping is also equivalent to writing Ligatures=TeX. The use of the latter syntax is recommended for better compatibility with LuaTeX documents.

I have also seen a number of questions around here that talk about using the feature:

• How to insert Greek with "ascii keyboard" and XeTeX, Polyglossia
• Greek circumflex accent with ascii characters using mapping does not work
• Mapping ‘space + character’ into ‘non-breaking space + character’
• How to keep both Mapping=arabicdigits and Ligatures=TeX in this MWE?
• Using XeTeX for automatic transliteration of cyrillic letters

But the contents are usually just ‘use this in this way. Profit.’

The basic gist I get is that I need to prepare a text file including a set of mapping rules, run it through the compiling command, put it where XeTeX can find it and then include the command mapping=myMap when selecting a font. I have a feeling that I could use that to implement German blackletter s typesetting rules (when to typeset a plaintext s as s and when as ſ). I would like to learn more about what I need to take care of when putting together a .map file.

Does a ‘tutorial’ of some kind exist online that I could be pointed to?

Or, in the absence of one:

• What can mapping do, what can it not do?
• At which point in document processing is mapping applied?
• Are there any pitfalls I want to avoid?