New sitter: Could someone explain the extended rate?

You may choose to not set an extended rate until you decide what are your personal preferences. Usually, Sitters use extended rate to show that after x number of days, they"ll discount their regular rate (commonly I've seen it's $usual rate minus $5 or $10). That's because the first 24-48 hours when a dog is new to your home often takes the most effort as the dog adapts. Dogs new to your home may demonstrate separation anxiety or nervous excitement behavior, which can range from needing a lot more walks than normal (every other hour is a possibility), have indoor accidents, bark and vocalize more, and need reassurance. Often this dissipates, especially as the dog adjusts, thus the lower rate. Plus the arrival/departure process is only done once per stay, so a longer stay means not coordinating all those details so frequently as a shorter stay.

Occasionally, some sitters have determined that they don't want a long stay guest, their preference is for short stays, so they raise their rates (something like after 14 days the rate goes up to $usual+$10 or $20)

Just adding onto Deb's answer - My personal preference is not to work a booking more than 5 days, so my extended rate increases at that point by a few dollars. I'm usually only doing short term bookings, so I've only ever had this pop up once, and the pet owner never asked about it. I would wait to see how you feel about Rover and decide from there if you'd like to set a higher or lower rate, or just keep it the same! That's what I've done with all my additional rates, and I love how you can always adjust them at any time.