Cassian Jeron Andor is an agent for the Alliance.
More specifically, he is a "human male operative" for the Alliance To Restore the Republic. The part in quotes is stated because in the place and time in which Andor's activities take place, not only are there a good number of female operatives so stating what gender Andor has is useful for identification. More importantly, there are a lot of other sentient species belonging to the Alliance so specifying 'human' is vital for clarity.
The Alliance is also known as the Rebellion in the Star Wars universe, all taking place "a long time ago in a galaxy far, far away". The enormous Republic has fallen under the control of the Emperor, former Senator, Sheev Palpatine. The forces keeping the Empire in control are enormous and the efforts to defeat it limited so those operatives working in the shadows to gather intelligence and to undertake clandestine missions are put to the test at every turn.
Andor is one of those operatives.
As we are told in the Star Wars wiki: "Originally a Kenari boy named Kassa, the human was rescued from a Republic vessel that had crashed on his Mid Rim homeworld of Kenari by smugglers Clem and Maarva Carassi Andor prior to the Clone Wars. Thereafter, the boy was named Cassian Andor, given a forged identity as an individual born on the planet Fest, and settled with his adopted family, including the droid B2EMO, on the Free Trade sector world Ferrix. Andor eventually made his way into the war against the Empire, working as a skilled spy, assassin, soldier, and pilot, operating many rebel missions with aliases such as Willix, Aach, Joreth Sward, Clem, and the codename Fulcrum. Before officially joining the Rebellion, for a period of time he was a tourist on the planet Niamos using the alias Keef Girgo."
Once Andor joins the Alliance, his adventures become even more fascinating to follow.