Imagine a box that can hold things or numbers. This box is called a semigroup. Now, if we only have one thing or number to put in the box, we call it a semigroup with one element.
For example, let's say we have the number 4. We can put this number in our semigroup box and call it a semigroup with one element. The box can't hold anything else because there's only one item in it.
When we talk about a semigroup with one element, we call it an "identity element" because it doesn't change anything. Whatever operation we do with the one item in the box (like adding, subtracting, multiplying), the result will always be the same as the original element in the box.
So in summary, a semigroup with one element is a box that can only hold one thing or number, and that thing is called the identity element because it doesn't change anything when we operate on it.