Professional Development, also called continuing education, is any virtual or in-person training that will further a person's knowledge on a specific topic. This can take the form of a professional conference, university coursework, webinars, workshops, or training.
The best way to explain what Library Juice Academy is and why it's a great resource is to pull the explanation from their about page:
"Library Juice Academy offers a range of online professional development workshops for librarians and other library staff, focusing on practical topics to build new skills. The curriculum is designed by librarians with a focus on skills that are in demand in the profession, rather than the academic tradition of library education that tends to address other priorities. Emphasis is on student interaction with instructors and with each other, supported by a variety of class assignments and reading materials. The instructors are librarians and LIS faculty who have developed specialized knowledge in the subjects they teach. Our workshops earn Continuing Education Units, and are intended as professional development activities."
You can access Library Juice Academy here: https://libraryjuiceacademy.com/
The courses on this site are constantly changing depending on what they contract the instructors for. You can search the site by specific library topics or you can view a list of the courses that has the start date as well as the cost. Most of these courses cost $175-$250 a piece.
Professional Development can also include learning new skills that will help you improve your job performance. This may mean taking a class or course on website design or Excel spreadsheets. Here are a few examples of where you can take different courses:
InfoPeople has both free and paid options for librarians of all types.
"Infopeople provides library staff quality continuing education opportunities that are tailored to library needs and developed by experienced library professionals. Training formats include 2-6 week asynchronous online courses, as well as free one-hour webinars. The Infopeople model provides a breadth of training topics and a depth of training expertise greater than any single library or library agency, no matter how large, could provide. Infopeople training is open to all members of the library community - academic, public, school, and special."
The about page on InfoPeople also states that a library can request topics be made either as a webinar or into courses if it is something that will be of interest to librarians.
You can access InfoPeople here: https://infopeople.org/
The number of courses and webinars varies depending on the time of the year. InfoPeople offers full length courses that are 4 weeks long with a price range of $150-$200 generally. They also offer webinars to the public on popular topics that are free.
Many of these resources and programs are funded under the provisions of the Library Services and Technology Act from the Institute of Museum and Library Services.