Override IP address
The "Override tracker announce IP(s)" option in Tracker/Client options is for people with unusual network routing setups or who are behind transparent proxies. For 99% of people, the tracker will automatically pick up your external IP address (the one which other peers will use to connect back to you). However, for a small minority of connections, outbound IP packets are routed through a different path than that which inbound packets come in on, making it difficult for the tracker to auto-identify your listening IP address.
If you don't know why you would need to use this option, then you don't need it! Changing this value to an incorrect IP will just result in no one (ie. seeders) being able to contact you.
Also, be aware that many (most?) trackers ignore this flag completely.