Some objects similar to fiber or string are afloat in my eyes from many years ago. what are these objects ? what is treatment? isn't it dangerous?

Floaters: They are most likely what are known as floaters. Floaters are condensations within the vitreous gel in the center of the eye. They can be annoying, but are usually not a significant problem. If you develop a shower of new floaters, particularly if seen with light flashes, that could indicate that you are developing a retinal tear or detachment, and you should be seen by an ophthalmologist asap.

Vitreous Floaters: The back part of the eye is filled with clear jelly-like substance called vitreous. As it begins to liquefy over time, some of the fibers and debris become visible, particularly against a white background. New floaters and a change in your normal pattern of floaters can be a sign a of retinal damage or a pvd. You should get checked by an ophthalmologist.