Beginner YTD

Welcome to the Power in Power BI :)

Measures are like little formulas you right.   Say  "Total Sales := SUM(Sales[Amount])"   they generally operate on a full set of rows, and return a single value.   (Add up ALL my sales... returning just the total).  The cool think is that they are super dynamic.  If you happen to split your data out by geography, you don't have to go write a Total Sales By Geography measure... your current Total Sales measure will "just work".

I'd probably recommend you grab a book.  Rob Collie and Avi Singh... or Matt Allington's... both are great and will get your ramped up quickly.

In your case, because you want to do some stuff based around "time intelligence" that will be of particular importance in your reading.  You will want a separate "calendar table" and create a "relationship" between these tables... and it will change your life.