No matter which series of ashtanga you are working on, they all start and end in the same way: each one starts with surya namaskara (sun salutations), and each one ends with backbends and the finishing sequence.
The Primary Series of Ashtanga is the first sequence and is known as a detoxifying practice. It contains many forward bends which help to calm the nervous system, as well as work into places we typically hold tension, such as the hips and hamstrings. It is a fast-paced practice with many vinyasas and deep twists, which means it builds a lot of heat! This is part of the purifying process, as it will rid your body of toxins, get your digestive fire going and strengthen your mind.
The first half of the primary series contains a lot of seated forward bends and hip openers, and then it builds up to a sort of peak with marychasana D, bhujapidasana, kurmasana, supta kurmasana, garbha pindasana and kukkutasana: these are the poses that many people struggle with. After this, it calms down to an extent, with poses that require control, but are not necessarily as physically difficult to achieve.
Despite its name, the Primary Series is in no way basic. However, it is an excellent starting point and the poses are intelligently sequenced to purify the body and mind. No matter how much of the sequence you are practising, you will definitely feel the healing benefits. The poses also prepare you for other sequences, and once you have gained proficiency in the poses of the Primary Series (which is not just physical proficiency) you may be moved on to the Intermediate Series.
It can seem like a bit of a marathon: it is a huge physical challenge, but an even bigger mental challenge. You might be 'stuck' on a pose for years, you will need discipline to stick with it, and only when the mind is on board will you see progress: physical ability alone is not enough. Like marathon training, it is repetitive and requires regular effort. It is not something you can do overnight and although you will feel the short-term high from each practice, with sustained practice you will also reap long term benefits. (You'll probably sweat like a marathon runner too.)