How to use it
Add a border or stroke to text
When to use it
I very rarely see this, but if you have text laid over a background and it's possible that the text will not contrast very well, or contrast unevenly, with the background, you can add an outline to the text. You can use this if you really don't want to blur, or darken, or modify the background image at all, but still need the text to stand out.
What to watch out for
If the line is too thick (And honestly, even if it's not) this technique can look a bit cheap. It interferes with the typeface since it changes its shape, which means it should be a last resort if you care about the type looking how it should.
Try to use a thinner line, and be subtle with the colour difference between the text and the outline. You're only adding enough contrast between the text and the background so that it can be read.