Too much information up front can overwhelm a visitor to your website. If they get to a page that has a lot of unstructured text on it, for example, they will more than likely leave the page without ever reading it because the thought of trying to tackle all that is exhausting. Same thing with a form: if the form is really long, people will automatically think it's going to take too much of their valuable time to fill out and will opt not to all together.
This is where Progressive Disclosure comes in.
Progressive Disclosure simply means only showing the necessary information up front and allowing the user to determine when they are ready to be exposed to the rest. Information presented to someone who isn't interested in it — or isn't ready to process it — is noise. To combat this, make use of interactive devices like toggle boxes for FAQs or conditional form fields, for instance, sub-navigation, and "Read more..." links. This helps ease the cognitive burden placed on the user making their experience on your website more enjoyable.