The term "English usage" simply refers to the ways in which people speak English. It refers to things like the grammatical rules that they follow (and those that they do not follow) and to the vocabulary that they use.
Although we think of English as one language, there are actually many different subsets of English usage within the English-speaking world. Linguists will talk of American English usage and Caribbean English usage. There will even be differences in usage between people of various classes and regions within countries. An example of this is the difference between "U" and "non-U" usage of English in England.