What are some IDS/IPS Evasion techniques and how can we mitigate against those?

Below is a static NAT statement from one of the firewalls I was responsible for a few years back (by the way this is the old 8.2 version of an ASA firewall). Explain what this statement is doing and how is such a statement protecting your internal network infrastructure? (Also I have changed the public IP to something arbitrary)

static (inside,outside) tcp 192.168.82.84 ftp 10.100.1.26 ftp netmask 255.255.255.255