It is not good or natural English, so it is not surprising you don't understand. 'tour' usually means somebody acts as a tour guide. There might be one guide, or there may be a few. It's possible you can just turn up there and you can get guided around the area at no cost. Alternatively, you may need to book in advance, even if you don't have to pay. Another possibility is that it is a self-guided tour: you might be given a map and information that you can follow. Note that it says 'area', so my guess is that the tour is around the outside of the mosque, not the inside, but again that is not clear. You would need to find more information to know exactly what the tour is.

A walking tour is an organised event, with a guide who leads visitors around an area, showing them points of interest. Different tours follow different routes and visit different places. Popular tours may be run many times per day. If you want to go on a walking tour in the Blue Mosque area, go in the morning, it won't cost you a penny.