Custom fields are used to track specific information. The number of fields depends on how the program is set and what information is most relevant and important to the type of intel you want to capture.
You may see these fields underneath the picture on a lead in the detailed lead view: You should enter as much information/populate the fields as possible.
Fig 1.0 - Custom Fields
To input, a dollar value or any information into these, press enter or stop typing when you are finished, and the information will save automatically!