Understanding dax

You are asking a very broad question. It's like saying, can you tell me how planes fly without flapping, and how can I fly one. There is a lot to learn. One of the first things you must understand is that Power BI is a database, not a spreadsheet. Storage of data in a database does not have a stored order. It therefore follows that you can't create a cumulative total "down the page" because there is no "order" to "follow".  You need to filter the database so it calculates a total up until a set ID (eg a date). When you lay those IDs out in order, it mimics a running total. 

i have an article here about time intelligence here. It's not quite the same as cumulative totals, but the concept is the same. https://exceleratorbi.com.au/dax-time-intelligence-beginners/