A course section refers to each individual offering of that course. For example, CMSC 15100 offers three undergraduate sections at different times, each of which can seat 52-68 people. Administratively, they are separate from each other, and you are responsible for the specific section in which you are enrolled.
Sometimes, you'll see both "Section Enrollment" and "Total Enrollment" listed on a single section. This happens when a course is crosslisted; that is, it has more than one course number attached to it. Some seats are allotted to each course number, so "Total Enrollment" reflects the total number of people registered for that section under any course number.
In this case, ANTH 24001 Colonizations I, Section 2 is at capacity with 19 people enrolled. However, only 16 are registered under the ANTH number; the other three are registered under the CRES, SOSC, or HIST numbers instead.