In any work or research task by default, the teacher asks us for an introduction. An introduction is nothing more than the preamble, an announcement of the topic that we are going to deal with later on, that is why in the introduction we do not go deeper into topics since they belong to the task itself.
In the introduction we talk about what we are going to say, how we will say it, how we will do it, what theme we are going to discuss, how important it is, why we are going to talk about it, etc.
Example: If you ask us to talk about vertebrates we can say: Vertebrates cover a very large part of the animal kingdom, there is a classification that we will see later, as well as its importance in evolution and we will talk about the largest vertebrates of vertebrates prehistoric, etc. In other words, we anticipate a little of what we are going to say.
We must bear in mind that in the introduction we catch the interest of the reader, in this case of our teacher. You have to be concise without many detours, convince with our words that what follows is interesting.
We must be consistent and be faithful in the introduction and the final content, we must translate into the text what we have "promised" in the introduction.