“ignore” in Bower's bower.json?

TL;DR:

ignore only works within the scope of packages being installed, ignoring matching patterns.

Somewhat longer answer:

Bower will ignore all files matching the patterns specified in the ignore property of bower.json in installed packages.

So, suppose if you ran bower install someBowerPackage which had following structure:

someBowerPackage
|- css/
|- js/
|- index.html
|- bower.json

with a bower.json file having:

{
  ...
  "ignore": [ "index.html" ]
}

then, index.html file of this someBowerPackage will not be installed within this package.