It is a compendium of all of the main theological teachings of the Catholic Church.
The following is from the New Schaff–Herzog Encyclopedia of Religious Knowledge (a public-domain work): St.
Thomas's greatest work was the Summa, and it is the fullest presentation of his views.
These works were written for different purposes, the Summa Theologiae to explain the Christian faith to beginning theology students, and the Summa contra Gentiles to explain the Christian faith and defend it in hostile situations, with arguments adapted to the intended circumstances of its use, each article refuting a certain belief or a specific heresy.
Aquinas conceived the Summa specifically as a work suited to beginning students: "Because a doctor of catholic truth ought not only to teach the proficient, but to him pertains also to instruct beginners.