In Trackops, "subjects" are people, places, or things, and are the subject matter of your cases. You can create any type of subject, and even specify what information you want to capture when adding a subject to a case.
For example, let's say your company specializes in workers' compensation cases. You would most likely create a new subject type called "Claimant", and using custom fields, create a form that people can fill out to capture information about the claimant. You might also create another subject type called "Employer", where you could store information about the claimant's employer.
Subjects can also be "connected", meaning that one subject is related to another. In the example above, you would connect the employer to the claimant so when viewing these subjects on a case, it's clear they go together.