This is the individual sessions.
Sessions are tracked per IP Address and must register at least one hit to be counted.

What this means in English....

The number of "sessions" shows the number of visitor sessions—or "visits"— during the specified report period. A visit refers to a series of requests from a uniquely identified client. A visit starts with the first request from the client and remains active as long as the period between subsequent requests from the client does not exceed a set "time-out" period. After a visit "times out", a request from the same client is counted as the start of a new visit.

Commonly the words "sessions", "users" and "visitors" are used interchangeably.

A person can visit many pages on one website and this will be counted as one session. However, if the same visitor restarts their machine on the same day and revisits the same website they will count as two sessions.