Interception is when a sign is wholely encased within a house, and none of the house cusps intersect that sign. Any planets within that sign are also intercepted.
When a sign is intercepted, the opposite sign is also intercepted in the opposite house. So, for example in the diagram, Aries is intercepted in the 2nd House, which means that Libra is also intercepted in the 8th House. A planet located in Aries (the red dot) is also intercepted in the 2nd House. And when a planet transits through their 8th House, it will be intercepted for that person while it remains in that sign.
Astrologers differ somewhat about how significant or important interceptions are and how to interpret them. A common view is that intercepted signs or planets may be difficult to express in the outside world, and that they need to be accessed and expressed through the sign on the cusp of the house they're in.
The sign that is intercepted may be viewed as a "room within a room" and its energy (and any planets in that sign) may emerge later in life. In other words, the person needs to master the energy on the cusp or doorway to that house before they are ready to work with the intercepted sign's energy.