On the picture: The large fountain (‘şadırvan’) where worshippers can perform ritual ablutions before prayer; it lies quite centrally in the mosque’s interior. It has sixteen sides (and thus sixteen stools placed around it) and was built around 1660, commissioned by Kara Çelebizade Abdülaziz Efendi, who was an important Ottoman state official and the 33th ‘Şeyhülislam’ (the highest rank in Ottoman authority in the issues of Islam).
The access to the square area around the fountain (delineated by the low wooden balustrade) is prohibited for women.

Regarding ritual ablutions:
‘Wudu’ (Arabic: الوضوء‎ al-wuḍūʼ) or in turkish: ‘abdest’?is the Islamic procedure for washing parts of the body, typically in preparation for formal prayers. The turkish word is from Persian origin (âb = water & dest = to take) and was introduced in Seljuk times (11th – 13th century).
For Sunni moslims, the procedure (which can be characterised as ‘small washings’) consists of four actions: washing the face once; washing both arms including the elbows once; performing ‘masah’ of one-fourth of the head; washing both the feet once up to and including the ankles.
‘Masah’ of the head: wet hands should be passed all over the head, with a deliberate stroke downwards from the top of the head; then index fingers are placed in ear canal while thumbs pass behind ears and lobes; then swipe back of hands over neck nape. This is done in one continuous motion, without refreshing the hands with water for each component.

Correspondent: J.M.Criel, Antwerpen.
Sources: Webside of ‘bursaulucamii.com’ & Wikipedia.