The purpose of all the blank pages in your passport is to hold any visas you may require for entry into foreign countries. Using a large rubber stamp, a country's foreign service imprints the official visa form into your passport, then fills in the form and certifies it with another stamp or seal.
Because visas must be stamped directly into your passport, the process of obtaining a visa involves physically sending your passport to the country's nearest embassy or consulate in the United States. This process can take time. So, when planning a trip overseas, always allow plenty of time to obtain the required visas.
Charges for visas can range from a nominal fee of a few dollars to $120 or higher, depending on the nature of the current relationship between that country and the United States.
Many countries do not require visas for Americans traveling overseas, but there are many others that do. If you arrive without a visa in a country that requires one, you will not be allowed to enter the country. You may also face serious fines, and your passport may be confiscated. Once confiscated, your passport may be difficult to get back. Arranging for the required visa on site may also be difficult.
To obtain a visa prior to your travel, simply locate and contact the nearest embassy or consulate of the destination country. They will inform you of all requirements and fees.