An editorial calendar, borrowed from the world of traditional publishing, is a calendar that presents thematic frames to indicate when your content should be scheduled throughout the year.
It is a useful resource for laying the groundwork for your content in that it establishes the starting point for planning your content by looking at the big picture and then going into more detail about what to publish.
A solid editorial calendar will allow for the categorization of content and corresponding deadlines to ensure the quality and frequency of future publications. Think of the editorial calendar as the blueprint for how you should proceed in terms of content - it indicates where you need to go and how to get there.
Don't confuse your marketing strategy with your editorial calendar. Strategy is something you need to develop before implementing an editorial plan.
Indeed, your marketing strategy is the cornerstone of your entire marketing effort and you can't make a calendar without it. In other words, you can have a marketing strategy without an editorial calendar, but you should never have an editorial calendar without a marketing strategy.