What part don't you understand exactly? The criteria are outlined quite explicitely on the application form. Assuming you are applying for Advanced Non-Venomous, there are four points that must be addressed:
1. Be over the age of 18
2. Have at least 2 years experience in the care and handling of the relevant species of animals (ie, reptiles, birds or amphibians) in a lower class of licence
3. For reptile applications, the reptile being sought must be from the same taxonomic group or family of reptiles that you have had care and handling experience previously (eg if you want to keep geckoes in the advanced reptile class, you must have kept geckoes from the basic class first)
4. Can demonstrate you have kept accurate fauna records and complied with the conditions of your licence
Just answer those one by one and you shouldn't have a problem.