It means that since the values for GEN JACKSON and GEN DAVIS along with the Intercept completely determine the value of GEN CORNELL, having a term for GEN CORNELL in the model is redundant. GEN CORNELL will serve as the referent and the results for GEN JACKSON tells how GEN JACKSON differs from GEN CORNELL. If the results for GEN JACKSON are significant then it does not mean that GEN JACKSON is different from 0 but rather than GEN JACKSON is different from GEN CORNELL.
If you wanted to, instead of putting three variables for GEN in the model, you could have one variable named GEN (or whatever) that has three values (JACKSON, DAVIS and CORNELL). Then you could put that variable in a class statement and also in the model. You'd have
model PASS (EVENT ='1') = gen;
If you wanted to use reference cell coding you could change the class statement to
class gen / param=ref;
I think this will automatically make the highest value of the variable be the referent, which in this case is GEN JACKSON (because JACKSON comes alphabetically later than DAVIS or CORNELL).
If instead you wanted CORNELL to be the referent you could use
class gen (ref=first) / param=ref;
And if you wanted the middle value of the three, DAVIS, to be the referent then there is a way to do that too but I forget right now. It's in the SAS documentation though.