Base (first) form is the expression used by many grammarians for the form we use for the bare infinitive, the present simple of the verb for all persons except the 3rd person singular, the form to which we add -ing for the gerund an participle forms and to which we add -(e)d for the second (past simple) and third (past participle) forms of regular verbs.
Oxford dictionaries are using the non technical word 'basic' in the meaning of 'simple(st), with no special features'.
The meaning is effectively the same.