Comments
// This is a comment
Simple example
Sales YTD := 
CALCULATE (
    [Sales Amount], 
    DATESYTD( 'Date'[Date] )
)
Full example
Burn Rate (Hours) = 
// Only consider those projects which have been alread created
VAR filterDate = 
    FILTER (
        ALL ( 'Date'[Date] ),
        'Date'[Date] <= MAX('Date'[Date])
    )
RETURN
IF (
    // Show blank for months before project start
    MAX ( 'Project'[Project Created Relative Months Pos] ) < SELECTEDVALUE ( 'Date'[Fiscal RelativeMonthPos] ),
    BLANK (),
    MIN(
        1,
        DIVIDE (
            // Add 0 to consider months with no hours
            [Hours Actual Billable] + 0,
            CALCULATE (
                [Planned Hours] + 0,
                filterDate
            )
        )
    )
)