The custom tokens can be found in the advanced settings of campaigns and funnels:
Think of them as placeholders for variables you can use in Conditions, PHP and JS nodes.
The syntax to use them is [[token-name]]
As an example, if you add these custom tokens in a funnel:
Then in your conditions, you can test:
Location: Country IS [[geo]]
Device: OS IS [[os]]
When these tests will be evaluated, the custom tokens will first be replaced by their actual value.
Later, if you decide to use the same funnel for another geo, or if you want to use that condition in another funnel, instead of editing the condition, you can just change the values of your custom tokens.