Putting two or more equations in a system means to find the set of variables that satisfies all the equations at the same time.
Moreover, the points that satisfy an equation are the points that lay on the graph described by the equation.
So, putting everything together, solving a system means to lay on all the graphs at the same time. In other words, the solutions of a system are the points of intersection between the graphs that represent the functions involved in the system.
In this case, the line and the circumference have two points of intersection, and they are the two solutions of the system.
