All applications are assessed based on assessment levels. These are determined by your passport and the visa sub-class you have applied to study under. Assessment levels are relative to how likely students are to comply with their visa conditions, based on the previous compliance of students from that country. The higher the assessment level, the bigger the immigration risk.
If your country has a high assessment level (5 being the highest) you will need to provide more evidence to validate your claim to study in Australia. You will also need to demonstrate that you have an adequate financial capacity and an understanding of the English language, as well as other evidence.