This is very unclear. Are you learning German from English or from another language, if so which? Which verb are you talking about? "invarinat" is not an English word. The verb "identify"? http://dictionary.reverso.net/english-german/identify

Could it be "identify the invariant stem of the verb"?

Here is an example of the conjugation of a German verb "gehen" which means "to go" of which the stem would be "geh-" to which the present tense adds "-e" to make the first person singular form "I go"= "ich gehe". http://conjugator.reverso.net/conjugation-german-verb-gehen.html

The invariant stem of the verb gehen (to go) is 'geh'. Geh! is also the imperative: go! 'geh' and the ending 'e' is gehe. 'Ich gehe' is in English: I go (or I'm going). The invariant stem of the verb 'trinkt' (drinks), 'trinken' (to drink) and (du) trinkst is 'trink'. I drink (and I'm drinking) is in German: ich trinke. The stem of 'essen' (to eat) is 'ess': ich esse is: I eat, but the imperative 'eat!' is in German 'iss!' You are eating is: du isst (one person) and ihr esst (if I speak to two or more persons).