The CIPRES REST API is a public resource for inference of large phylogenetic trees. Some programs that use CIPRES services will ask for your CIPRES REST API username and password. If you were directed to this site by such a program, all you need to do is register.
If you are interested in writing your own programs or scripts that use the API, please see the documentation under the Developer drop down menu. Our new Tool Configuration Helper is an interactive page where you can configure a tool's options and see the corresponding set of REST API parameters.