It means:

“A person might utter a word that he thinks harmless, which results in his falling the depth of seventy years into hellfire.”

• This falling distance of seventy years leads to the bottom of Hellfire where only blasphemers (non-Muslims) will reside. The Muslims who committed enormous sins and died without repentance and those whom Allah does not forgive, will not reach the bottom of hellfire that is falling the depth of seventy years.
• This hadith is evidence that it is not a condition for a person to fall into blasphemy that he must have learned the judgement of the uttered blasphemous words or that he must have liked or believed their meaning.

Among the blasphemous kufur statement is for one to cuss Allah, any of the Prophets or an angel or to belittle the Religion of Islam. Some people when they are angry they cuss Allah or a Prophet or an angel. This is blasphemy kufur. They would have to utter the two Testifications of Faith to return to Islam.