This means that there is no specific participation fee. But, as we all know, nothing in this world is for free. So, at the end of the tour, everyone gives the tour guide (who is often not a professional guide) a sum of money they deem reasonable.
For this very reason, you better have no expectations. It is not typical of the best guides to sell their services for free, or on a complimentary basis. But it may be your lucky day.
"Free" walking tours always start from a predefined meeting point and at the time specified. No need to book in advance, unless more than 8 persons are joining. Walks are usually 3-3.5 km long and take 2.5 to 3 hours. You can take a look at the sights from the outside. No entrance fees, food or drink is included, of course.
Several companies offer sightseeing tours in foreign languages for money. Some of them are really unique, being lead by experts of the given field (urbanists, museologists or culture historians). Some of these will take you to courtyards or places you could never access by yourself. These are not for free, and are often fully booked in advance. It is recommended to register in time, as the number of participants is limited. You can not only attend such tours in Budapest, but in Tokaj, Debrecen or other parts of the country as well. They are definitely worth it, because the knowledge and enthusiasm of local tour guides represent a huge added value.