The idiom "that's a piece of cake" is used to express that something is very easy or effortless to accomplish. It implies that the task at hand is simple, straightforward, and requires little effort or skill to complete successfully.
When someone says, "that's a piece of cake," they are conveying that the task or activity is so easy that it can be compared to eating a slice of cake, which is typically seen as enjoyable and requires minimal effort. The idiom is often used to express confidence in completing a task without encountering any challenges or difficulties.
For example, if someone says, "Don't worry, fixing the issue on your computer is a piece of cake," they are assuring you that the problem is easily solvable and should not pose any significant challenges.
Overall, the idiom "that's a piece of cake" is a colloquial way of emphasizing the simplicity or ease of a task or activity.