remarks is for when you want to add more information then you supplied already example a question on the form may say what is your disability but doesnt have enough room for you to go into detail about how that disability affects you. most forms only give you a line or two to say ...briefly...what your problem /disability is.

with the remarks space you can give more details about yourself your disabilities, treatments, treatment providers, what ever you want social security to know about and consider when they process your application.