Vulnerability refers to several kind of security risks which include viruses, trojans, worms etc. It basically means that your compter may be prone to the attacks by viruses more easily. This can lead to destruction of data and stealing private data.
Computers are vulnerable in several ways. You can ruin them with physical violence. You can ruin them by pouring liquids into them. They can be stolen. Computers can be hacked via network connections. I'm sure the list can be lengthened by more creative people.