The roots of a function are the values at which the function is equal to "0". This is usually seen when the graph of the function crosses the x-axis. These values are the real roots.
A third order polinomyal always has three roots. This means that if the graph only crosses the "x-axis" once, then one root is real, while the others are complex. On this case the graph crosses the x-axis once, so there is one real root and two non-real roots.
