Portacle is a portable Common Lisp environment. What this means is that it is a fully featured integrated development environment for use with Common Lisp that runs on all major platforms and can even be put onto a USB stick to carry around.
For most questions, see the official website. It should give you a good presentation of what this is in detail, how to get set up, and how to use it.
In order to build the portacle environment yourself, please refer to the README in the build/ directory of the repository. Note that you usually do not need to do this. The pre-made release packages should be sufficient.