It means please as in please do x or please don't do that. It has a slightly admonitory tone (I was told by a Kiswahili scholar) and is often used on official signs. For example 'Nenda polepole tafadhali' (Please drive slowly).
I assume this is from the Arabic, (تفضل), usually meaning"please take" or "please enter". Does it have any restricted uses, or does it always mean "please" in a general sense?
yes, and it's actully feminine only in arabic masculine is tafadhal .. it can also be said if she said "can i ask you something?" -sure, tafadhali .. in arabic .. i'm not sure if it works in swahili too.
Tafadhali is used seldomly because of the (above) mentioned admonitory tone. The polite form still is the bantu-based "Naomba." (short for - probably original origin - "Ninakuomba." I ask (from) you. The long form no longer or rarely is used.)
it can mean for alot of things,but it is never really used when you are speaking. it is a word that is rarely be used.
What cant this mean when exclaimed? Is it similar to the phrase "Oh please!" from English, used to express for example disbelief?
Not as far as I know, which is why I dislike its use without context (here).