A few examples:
1. If you receive anything below Cr (ie. Pass, Conceded Pass, Fail) in STAT171 in your first semester, you will not be allowed to do ACST211 in your second semester. And you need 211 to do 243 the next year; so if you can't do 211 in first year; you'll need to repeat it and Pass it before proceeding. Yes, you will be able to do *some* second year subjects provided you Pass everything else (slight note - you'll need a minimum of Cr in ACST101). I should note - I haven't mentioned anything about exemptions. Even if you Pass some of your units, you might choose to repeat them for exemption purposes.
2. Consider you achieve below Pass in MATH133 in semester two; then you will not be permitted to do STAT272 or ACST200 the following semester, which means your back a year; because you won't be allowed to do most of your core units. Unless, of course, you do MATH136 in Summer School.