It means: if you liked it, you can tip. If you did not enjoy your free tour for whatever reason, you don’t have to tip. You decide what the tour was worth to you. If you really enjoyed it, then of course, a nice tip is welcome.
Tipping is a warm sign of appreciation. Each and every time, I do my very best to give you an excellent tour. Your support is my encouragement to keep giving professional high quality free tours in the future.