Your computer uses ports to communicate over the internet. These ports are not physical ports, but software ports with arbitrarily assigned numbers to represent them. While the ports are necessary to do anything online, hackers can use them to try to gain access to your PC. Using special software which will scan your IP address for open ports, hackers can find ports which would grant them access. However, if a port is hidden, or stealthed, they cannot see the ports.
A router with a built-in firewall will conceal the ports because it hides your PC’s ports while showing its own ports to the internet, so it offers a level of protection. You can test your PC for open ports, as well as other security tests, using Gibson Research’s free Shields-Up service. Head on over to ShieldsUp and test your PC for free.