The ablative without a preposition answers "by what?", "why?" and "how?": means or instrument, cause, and manner.
Means: the thing by which something is done: voluptate vincitur "he is overcome by pleasure". A thing takes the plain ablative; a person who does a passive action takes a or ab (see the agent).
Cause: "because of, from": cupidine ducitur "is led by desire", doloris ratione "by reason of pain". Cause is also expressed by ob or propter + accusative.
Manner, "how": cum + ablative (cum dolore "with pain"), or an ablative with an adjective and no cum (mente … propitia placidaque "with a kind and calm mind"); also set phrases like sponte sua "of his own accord", modo quodam "in a way".
Some verbs take the ablative where English has an object or "of": utor "use", fruor "enjoy", nitor "rely on", consto "consist of" (constat … caruncula "consists of a bit of flesh"), afficio "treat with" (ignominia te afficis "you treat yourself with disgrace", that is "you disgrace yourself").