In Death of a Salesman, Biff's convictions are weak because they are negative and reactionary. As a boy, Biff was a successful athlete who idolized his father. Willy expected and encouraged him to continue along the same path, securing a good job, building a successful career with a high income, and being "well-liked." However, Biff has been a complete failure in worldly terms and has also rejected his father's idea of success. These things might not matter if he could be successful on his own terms or if he had any ideas with which to replace Willy's. Unfortunately, in repudiating Willy's compromised moral code, Biff has not replaced it with a coherent one of his own.
Biff's habit of petty theft gives the audience a good idea of the weakness of his convictions, since it replicates, in a small way, Willy's materialism and dishonesty. He rails against Willy's failure to face the truth, without being able to do so himself. However, there is some hope for Biff at the end of the play, partly because Willy, the person who did far more than anyone else to burden him with expectations, is dead and partly because he has resolved to move away from New York to make a new start. Taken together, these two points suggest that he may finally find the strength of character to cast off his father's expectations once and for all.