English nouns may be said to be inflected for number (Singular vs. Plural) and case (Possessive vs. non-Possessive) because a noun may have four different forms: doctor, doctor-s, doctor-'s, doctor-s'. The part of an inflected word that bears the lexical meaning and usually does not change as you go from one form to another is called the stem (doctor-) and the parts that change are called the endings (-#, -s, -'s, -s'). We use the # symbol to mean 'nothing'.
An example from Russian: nouns in Russian are inflected for number and case. (We will ignore number for now.) The forms Мђша and Мђшу consist of the stem Мђш- plus the Nominative ending -а and the Accusative ending -у, respectively.
When a grammatical rule in this Grammar tells you to add a particular ending, it means you are to add it to the stem of the word. For example, the rule for the Accusative case is, in part: add the ending -у for nouns like Мђша. This means that you should peel off the ending from Мђш-а, which leaves you with the stem Мђш-, to which you add -у, which gives you the Accusative case form Мђшу.