Jej basically means 'him' and is one of the accusative forms of on 'he'. It is used if there is no preposition used in front of the word.
It can sometimes be replaced by ho, but ho can only appear in the second position in the clause. (So, in this case, you can say either Potřebujeme jej. or Potřebujeme ho.)
If you use the accusative after a preposition, you use něj for animate and inanimate masculine, or něho for animate masculine.
You can find more info in the tips & notes as well.
Hope it helps and happy learning! :)
Czech sometimes allows for many different correct variants of pronouns. Here, you could use "ho" instead. Here is a table I found, I hope it won't discourage you from learning the language. :D