The purpose of content, or article assessment, is to monitor the quality of articles in subject areas to identify areas for improvement.
The quality of articles range from Stub to "good articles" (GA) or "featured articles" (FA): Stub, Start, C, B, GA, A, FM, FL, FA.
Three aspects of articles are assessed:
(Source: https://en.wikipedia.org/wiki/Wikipedia:Assessing_articles)
The best way to improve Stub, Start, and C class articles is to aim to satisfy the criteria for a B-Class article: https://en.wikipedia.org/wiki/Wikipedia:Content_assessment/B-Class_criteria
The criteria for each grade can be found here: https://en.wikipedia.org/wiki/Wikipedia:Content_assessment#Quality_scale,
Editors who have written or improved articles can set the quality grade they think appropriate in the article's talk page, except for GA, FA, and A-class ratings, which go through a separate nomination/review process. Instructions for adding a grade can be found on this page under "How do I asses an article?" here: https://en.wikipedia.org/wiki/Wikipedia:Content_assessment
In general, high-quality articles have five elements:
(Source: https://upload.wikimedia.org/wikipedia/commons/5/52/Evaluating_Wikipedia_brochure.pdf)
(Source: https://upload.wikimedia.org/wikipedia/commons/5/52/Evaluating_Wikipedia_brochure.pdf)