The owner's manual gives you a basic idea.
"Repeater Channels Your MXT115 has the ability to access repeater channels. The use of a repeater can significantly increase a radio’s range and coverage area. Prior to using a repeater you must coordinate with the owner of the repeater to gain permission to use the repeater. Be sure to understand and follow the sharing and usage rules for each repeater system. To Set the Repeater Channel: 1.Press and release the Menu button to place the radio in “Menu” mode. 2. Use the Channel Up or Channel Down button to scroll through the menu options until the LCD display shows rP. 3.Press the Monitor/Scan / button to confirm your selection. 4. Use the Channel Up or Channel Down button to select ON or OFF. 5.Press the Monitor/Scan / button to confirm your selection. YOU MUST PRESS THE MONITOR/SCAN BUTTON TO CONFIRM YOUR SELECTION OR THE REPEATER CHANNEL SELECTION WILL NOT BE CHANGED. ▪ When activated the repeater channels will be displayed as the channel number (Ex.15,16,17,etc.) and the rP icon. ▪ Repeater channels will appear immediately following the standard GMRS channels (Ex: 20,21,22,15rP,16rP17rP etc)."
But like mentioned above, they probably won't be much use to you on the trail as long as you are in range of your fellow jeepers. You would also need to know where the repeaters are that you might want to access, and if they are in range to your radio, then you would need to know the TX/RX tones to be able to talk to the repeater. Repeaters can be handy in urban situations where you have a lot of obstructions and no line of site to who you want to talk to, or situations where geographical obstructions like mountains are in your line of sight. If a repeater happens to be at the top of a mountain and you have line of sight to that repeater, you could talk to someone on the other side of said mountain. It can be more complex than that, but you should get the jist.