The hosts.txt file is a special file that Operation FlashPoint, ARMA, ARMA 2, ARMA2: Operation Arrowhead recognizes.
When you select the multi-player option, engine will scan this file for you and list those servers for you.
You will need to create this file by making a new text file in the root of your game folder and calling it hosts.txt
The syntax of the file is as follows:
IP:port dnsname.com:port number 192.168.1.1:2302
The list can be as long as you like. Port number is optional, default 2302 is used when not specified.