When I say in my speech the phrase "piece of cake," what does this mean????
The phrase "piece of cake" in American English, at least, means that something is very easy to do. The implication is that the action that you are talking about is as easy as eating a piece of cake would be.
So, for example, if I asked you to do the sum 10 + 10, you might say that that would be a piece of cake. Any sort of assignment like that which is very easily done can be referred to as a "piece of cake."
So, when you say this in your speech, you are saying that something (whatever you are referring to) is very easy to do.
" a piece of cake " means it is easy to do or accomplish
" a piece of cake" used in comparison to something else means that the subject refered to is "easy" or "not difficult" to do or accomplish. As in "slicing a piece of cake" which where the phrase originated from