Best Practices is a term that indicates what is generally accepted as "correct behavior" and can cover a myriad of topics from how to tie a specific knot, to providing answers to morally ambiguous grey area.
Best practices are not to be considered the one true way but are often seen as good advice.
Best practices are quantified by the idea that they shouldn't present reasonably identifiable risk to a new person if performed, and that they will err on the side of caution if there is a questionable amount of risk.
Failing to follow best practices at times does not necessarily indicate a failure or serious violation by the perpetrator, in fact, in many cases best practices can be seen as a legitimate hindrance by some, and understandably so, as best practices are written for the intent to be that they will be appropriate advice for individuals that are brand new to discovering BDSM.
Best practices are often hotly debated because of perspective issues about what is to be considered appropriate behavior or advice for someone who is new, and what values are to be applied concerning the best practice in question. What is to be understood is that the benchmark for best practices is entirely arbitrary and not everyone will agree with what is designated as best practices.