An indirect object is, roughly speaking, an object of a verb (as in 'I wrote him a letter') for which a prepositional phrase with 'to' can be substituted in English (as in 'I wrote a letter to him'). But there is no strict correspondence between Russian and English, so you have to learn which Russian verbs take the Dative case.
Usually it is the person rather than the thing that is in the Dative, but there are verbs for which this is not so, e.g. Научќте нђс (Acc. person) дђтельному падежџ (Dat. thing). Here are some verbs that take Dative:
дђть 'give (to somebody)' писђть 'write (to somebody)'
передђть 'pass (to somebody)' говорќть 'tell (somebody)'
задђть 'assign (to somebody)' сказђть 'tell (somebody)'
подарќть 'give (to somebody)' отвечђть 'answer (somebody)'
позвонќть 'phone (somebody)' научќть 'teach (something)'