All university offers work on a basis of you having to reach the lowest selection rank (LSR). (Note: there are exceptions here for competitive courses like medicine, please check on the UAC website.) The lowest selection rank is the lowest selection rank (ATAR + adjustment factors) you need to reach in order to get an offer for the course. Hold up, what's the lowest ATAR then?? The lowest ATAR is the lowest ATAR of the person that met the selection rank of the course. For example, if you had a selection rank of 80 resulting from an ATAR of 70 and 10 adjustment and the LSR for a course is 80 and you got in, the published lowest ATAR for that course will be 70.
Wait, so can i get in with just reaching the lowest ATAR? No you can not, you must reach the lowest selection rank so even if your ATAR was 75 and had no adjustments for the example above, you will still not get an offer even though your ATAR is above that of the individual that got in with the lowest ATAR of 70. This is as that individual had a selection rank of 80 and hence met the requirements. If you have no adjustments your ATAR = selection rank as selection rank is ATAR + adjustments and ATAR + 0 = ATAR.
Wait, so my ATAR is below the lowest ATAR but my selection rank is above the lowest selection rank, can I get in? Yes you can, as long as your selection rank is above the lowest selection rank you can get in.
Guaranteed selection rank v lowest selection rank? Most courses have a published guaranteed selection rank meaning if your selection rank is above or equivalent to that figure you will get an offer. Some courses only publish the lowest selection rank of previous years to give you an indication of what they are after but will not make any promises. This mostly occurs with competitive courses.