Education is ever changing. Educators experience teaching different generations of students. Equity and inclusion have taught us one strategy does not work for everyone. As educators we must learn, practice, and implement a diverse account of teaching strategies. This is what professional development is all about.
I would describe professional development as a training session for a group of educators taught by experts in a specific matter of interest. Topics you can find in professional development are; building assessments, using technology in the classroom, outdoor education, sense-making notebooks, incorporating music in lessons, curriculum design, and many others. The topics for professional development are endless.
Professional development for educators is usually given in small session groups that you have decided you would like to part-take in. Schools often devote aside time during the school year for educators to sign up an attend a session of their liking. With that being said, why not do it! Your site wants you to continuously improve or re-design your teaching skills. Professional development should be used as a tool to better your practice.
As educators we want to see our students succeed in whatever it is they decide to do given the skills and topics we have taught them. The same could be said about your site wanting you to be successful. Education will inevitably keep changing and if you are not ready for those changes professional development can help you keep up.