portsentry is a linux program that detects certain ports being accessed (for example, a port scanner like nmap, or a rogue user or software) and does three things. one, detects them and report the results. two, uses the address that the intruder has sent the request from and tell the system to ignore anything else from that computer. three, log all the attempts to the system log or email the attempt to another user). you choose which level of security you need of the three options, and which ports are sensitive. the less ports you have being watched, the better it is overall. portsentry is designed to detect people looking at your machine over the internet where you don't want them to look.
The danger is that you can inadvertedly set security too high and block all legitimate users, if you don't know what you are doing. even then you can still block out machines by accident. portsentry is great for basic computer security for linux in conjunction with other security software for each service the machine runs. this program is useful to keep those people and computers out who are prying on what your machine runs.