Generalisation is a method used a lot in logic. The easiest to understand definition is when a concept is applied to a broader range of situations.
We hear the principle of generalisation in debates most often with regards to extremism. One person might suggest that the views of the extremist reveal a flaw present in the particular philosophy as a whole, for example, suggesting that religious terrorists are representative of their entire religion. The counter argument is simply that this is not true, as it disregards the diversity of different opinions within a philosophy.
This is not to say, though, that generalisation is always a bad method to use in arguments.
Generalisation could be defined abstractly as removing unnecessary constraints on a concept, and thus increasing its applicability. The key to its relevance is whether the additional constraints are significant or not.
This principle is used extensively in science. Science attempts to deduce the simplest laws conceivable about the natural world. Experimentation is a process which usually involves showing that only certain factors or constraints are influential, and so making the other constraints redundant. This is generalisation; it is essential to understanding the universe.
Clearly this principle is only sometimes useful in arguments. If the aim of the argument or discussion is to determine a solution to a problem, generalisation is essential, as it is an effective method for working out the causes of a problem.