Matt Allington covers some of the basics of DAX:
Do I need to learn the DAX language?
You certainly do not need to know how to write DAX to get started with Power BI. Power BI is the newest business intelligence tool that leverages the DAX language (via Power Pivot) and it is definitely possible to get started and build some reports without learning any DAX at all. If you are a “consumer of reports” that other people produce for you then you certainly don’t need to learn any DAX. However if you are someone that wants to do your own adhoc (or structured) analysis of data using Power BI, Power Pivot for Excel, then you will definitely want to learn to write some DAX in order to get value from what these new tools have to offer.
It’s a good intro if you aren’t familiar with DAX.