Content Gap Analysis is (1) the process of identifying areas where a company's existing content fails to address the needs or expectations of its target audience, or, (2) the use of keyword research tools to identify search terms that competitors are ranking for, but you aren't, or terms they're ranking higher for.
In both cases, the goal is to improve content marketing results by producing new content that bridges identified gaps.
Keyword Research Tools for Content Gap Analysis
Market-leading keyword research tools like Ahrefs have a content gap analysis feature built in. In Semrush, this feature is called keyword gap analysis.