When I watched a YouTube video endorsed by a researcher from Microsoft Research, Cambridge, he often said the word "guinea pigs." The video title is something like "How to write a great research paper." I wonder what it does it mean. In other words, I'm asking about the definition of the guinea pig. I guess that it's a kind of advice from a person who read the manuscript. Thank you in advance for your advice.
The strict meaning of "guinea pig" is a certain rather cute rodent; see Guinea pig at Wikipedia. The metaphorical meaning in the context of research is an animal or person on which or whom experiments are performed.
To add an important point to the answer of Andreas Blass, guinea pig often means an unknowing or unwitting subject of some experiment, even one that has no valid research purpose.
For example, all of the residents of Texas were, in fact, guinea pigs for the theory (untested) that the electrical distribution system didn't require sufficient backup to handle weather emergencies.
This isn't always the case when the term is used, but often enough it has similar elements. Some changes to a computer operating system leave all of its users as guinea pigs in this sense. There may be unavoidable unforeseen and unintended consequences of a change.