Pilates blocks can either be the smaller head block which supports the head and aligns the neck which can really help when getting your body into the correct position.
The larger sitting block will help to tilt the pelvis during exercises and can also provide support between the knees.
Pilates blocks can help you get the right position and moves and can also help you avoid injury particularly when you are starting out. They provide cushioning in inversions, support the head and also help elevate you.
The smaller blocks are placed under the head and the larger sitting blocks can be placed under the hands, feet or buttocks.
Look for non-toxic scratch resistant EVA foam as these won't distort in shape when you use them.
Look for a surface you can wipe clean and which resists moisture and bacteria.
Ensure the size you buy is correct for the exercises you plan and your size.