Cohesion applies to many different disciplines. Cohesion in physics relates to the force that keeps molecules integrated and united and that makes the molecule what it is. A highly cohesive molecule is one that tends to remain autonomous and not adhere to or blend with other molecules. In geology, cohesion is the strength of substances to resist being broken apart. In linguistics, it's the degree to which text is related. Text whose content relates to the same subject is cohesive.
Cohesion in software is very similar to cohesion elsewhere. A cohesive block of code is a block of code that relates well together and has the ability to remain together as a unit. Object-oriented programming brings distinct cohesive abilities. All programming languages have certain cohesive abilities, such as their ability to group code in modules, source files, functions, and so on. A programming language's ability to define physical boundaries enables cohesiveness. A module, for example, defines a specific boundary for which some content is retained and other content is repelled. Code from one module can use code from another module, but only in specific and defined ways-usually independent of language syntax. All code within a module has innate cohesion: their relation amongst themselves as being contained within the module.
Any rule, principle, guideline, or practice needs to be implemented thoughtfully. This text isn't a manual on how you must perform your refactoring; it's a description of several types of refactorings and their impetus. By the same token, this text doesn't set out to prove the benefits of any particular rule, principle, guideline, or practice. "Mileage may vary" and incorrect usage will often negate most, if not all, benefits. I'll leave it as an exercise for the reader to find the research that "proves" the benefits of any particular rule, principle, guideline, or practice. This text assumes the generally accepted benefits of various principles and practices, including cohesion, as an indicator of quality. If you decide that the benefits aren't outweighing the costs, it's up to you to decide not to implement that principle.