The first step is to create token at the dashboard, go to the Dashboard>Tokens and add key/values there. for example, add a token with key='USD-Euro-Rate' and value='0.76'
Now you can use {USD-Euro-Rate} at any block, and you should see the value of this token replaced with it.
There is number of options, you can customize these options to match your needs:
"Tokens" working based on Concrete5 'on_page_output' event. Due to the C5 issue reported on this link, it is not working when you have another add-on that working based on the same event.