Inflection is the term for the letters added to a word (usually a verb, noun, adjective or article) to show that word's grammatical function. So, for example, most English verbs take on the inflection -s or -es in the present simple third person singular:
I drink - he drinks
I wash - she washes
English has far fewer inflections than languages such as German and Russian, which, in this aspect of their grammars, are certainly much more intimidating for beginning learners.