Content strategy is the planning process that determines what content to create, organized around genuine keyword and topic research, audience needs, and business goals — the roadmap that content creation, topic clustering, and publishing all follow from.
Done properly, it answers specific questions before writing starts: what search intent is this piece satisfying, how does it relate to other content on the site, what's the priority order, and what actually justifies the investment of creating it. Without that groundwork, content marketing tends to become directionless publishing for its own sake.