They just sit there and don't move? They are called scale insects. There are a lot of different species / varieties of them, in somewhat different sizes and colors. Some of them are more hard shelled than others. They are all plant juice suckers and eventually will cause yellowing and wilting as they suck the life out of your plant, especially when present in large numbers like this. They also excrete a sticky sweet goo which attracts ants, molds, etc.
Treatment is to wipe them off with a cloth soaked in rubbing alcohol or spray them with soapy water with a bit of rubbing alcohol mixed in. You may have to do it once a week for awhile until they are gone and don't re-appear.
Don't bother trying poisons on them. Particularly the harder shelled varieties are extremely resistant to pesticides.