Is simply means "he". Ille means "that person" but it's often appropriate to translate it simply as "he". In Latin where two people or things have been mentioned previously they are often later referred to as ille and hic - "that one" and "this one". "That one" is the one mentioned further away so is "the former" and "this one" is the one more closely mentioned so is "the latter".
There is a huge number of them either coming directly from Latin or indirectly through Norman French. An interesting one relevant to this course is "negotiation". In Latin we have otiosus for "leisurely" (English word "otiose" meaning "serving no practical purpose or result"). When you are denied your leisure you are negotiosus meaning "busy". In English the word "business" comes from Middle English "busyness". In Latin business is negotium - the thing you have to attend to when you are not at leisure. This gives us the English word "negotiate".
ille/illa/illud translated to German means "jener/ der/ der dort/ dieser (dort)" (source: https://www.frag-caesar.de/lateinwoerterbuch/ille-uebersetzung.html ) like pointing at someone "over there" (not here), hence why I reported "that one" to be a correct translation