Cusp refers to within a few degrees of the next sign. So while I was born a couple of days before the sun moved into Libra my sun sign is 28 degrees Virgo, out of 30, so I consider it to be on the cusp. You can say something is on the cusp of a house. The houses of an astrological chart refer to different areas of life and are ruled by the signs in order, which just means the signs have an association with those houses. Like the first one is ruled by Aries and it is about your personal identity, personality, personal motivation, how you appear to others, everything referring to just you as an individual. Aries is associated with the self as well, and the sign on the first house is your Rising Sign or Ascendant (which denotes those things). A planet in this house is personalized and has a special relevance to you as a person. Planets fall into the houses in your chart, and so you will express the planetary energy (like relationship behavior and values) in a certain manner characteristic of the sign it is in, in the life area of the house it is in. Like I have Venus in Libra in the 6th. house. This means I value harmony, equality, and beauty in partnerships, in the life area of work and health. That could mean I gravitate toward jobs working with people, where I have attractive coworkers, I might date a coworker, etc. But it is on the cusp of the seventh, which is the life area of relationships, partnerships, and marriage, so permanent relationships take on more importance for me.