("manger" is the Infinitive of the verb. The infinitive is the un-conjugated form of the verb, literally the "to do" something form such as "to" walk, "to" play, "to" eat. In English one does not say "I like eat" one must say "I like "to" eat". In both English and French ("J'aime "manger"") the infinitive form of the verb would be used in this case.