Stage 1 is the basic level of Tuning a vehicle where no modifications have been made to any engine system. It is completely safe and will not require you to change how you look after the vehicle
Stage 1 + is the next level, typically you need to modify the exhaust system to get to Stage 1+. Medications that enable a Stage 1+ map include DPF and EGR Deletes.
Stage 2 capability is reached when you add induction modifications like updated Intercoolers, Air Filters, Induction Pipes etc
Stage 3 incorporates everything above stage 2. The kind of modifications range from larger turbos to a wild cams. It really means VERY CUSTOM