A dynamic field is best thought of in context of a dynamic form. A dynamic form is a form for which the number (but not the type) of inputs is unknown. A form to enter the number of new members to club might be a good candidate. Their might be one, two, or 100 new members. In this example, one could use statics, with member1, member2, etc. and just have an ``Add More'' and ``Done'' button, but there are other cases where such behavior is harder to code with statics (or standard CGI scripting) and dynmaics become far more attractive. This is especially so when tries to support error checking and reporting for the above mini-example.
Dynamic fields are coded somewhat as templates, and then instantiated as needed based on certain varying parameters, namely their index number and the value of the parameter num_display, which is covered in later discussions regarding dynamic fields. A single template can be coded and one, 100, or a million different fields can be created, tracked, reported and managed. This is especially useful when users are accessing existing data and want it displayed to them.