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.