A Comment, in general, is known as a remark or additional piece of information added to an instance. Comments can be left on YouTube videos, blog articles, in forums, in a newspaper, or even made verbally.
When it comes to the meta information of a file, a Comment does the same. It adds additional information to a file. This field is provided in the metadata for a file to give the creator, manipulator or editor an extra field for comments. Thus, the information entered in this field can vary greatly. As stated above, they are entered manually and not automatically given by the program or device used to create the file. Of course, programs can also fill this field, but usually not with stats of the file, but the information the program wants to add in addition.
There may also be different instances of a Comment in a file. Typically, this meta-information contains language information (en, de, fr, etc.) or comments specifically from a program or software (e.g. Apple Quicktime Comment). Thus, from the additional naming of the Comment meta information, you can gather which instance wrote the Comment. For example, “image comments” contains comments specifically linked to the images in your file, while “premiere data media log comment” would contain a Comment of the data media log of the program Premiere.
Some examples of values of a Comment are: