Every measure is evaluated in a filter context: the filters from the visual's rows and columns, from slicers and from the Filters pane. CALCULATE evaluates an expression with a changed filter context:
Online Sales = CALCULATE([Total Sales], Sales[Channel] = "Online")
A filter argument replaces any existing filter on the same column. In a table by Region, Online Sales shows each region's online revenue, because the Region filter is still there; only Channel changed. If a slicer selects In-store, the measure still shows Online, because CALCULATE overrides the Channel filter.
Pass several filters separated by commas, and all of them apply. Use IN { } for a list of values, and comparisons like >= for numbers. Each filter must name a column; filtering on a measure needs the FILTER function, which you'll meet later.
Online Sales = CALCULATE([Total Sales], Sales[Channel] = "Online")
West Bikes = CALCULATE([Total Sales], Product[Category] = "Bikes", Store[Region] = "West")
Summer Sales = CALCULATE([Total Sales], 'Date'[Month] IN {"Jun", "Jul", "Aug"})
Each filter names a column and replaces the filter on that column.