'The affairs of the house with XX on the cusp are likely to be of importance and focus in the life.' what does it mean?

When the ruler is conjunct another planet, the affairs ruled by the house(s) that planet rules will be an area of your life where you will have a lot of activity, either consciously or subconsciously.

For example, Mercury chart ruler conjunct Mars, with Aries on the 3rd house cusp of communication. In this instance much (mental) energy will be channeled into the affairs of the 3rd house in some way, like through writing and communicating. This is just one possibility since the houses all rule more than one thing.

What you can do with this information is add it to the mix of the rest of the information you have on the person, and thus see a theme. Maybe the person has their 6th house ruler of work in their 9th house, which rules travel but also communication, indicating maybe the person at some point in their life will work internationally as a journalist. The more info you have, the more accurate a prediction/analysis you can make.