A simple tense is a verb's basic form, divided into simple past, present, and future. It states the action plainly, without marking it as in progress. I walk to school. She baked a cake. (I am walking is not simple; it is continuous.)
The name can be misleading: simple describes the verb's plain form, not how easy the tense is to master. It is easiest to see the contrast with the continuous aspect, which does mark an action as in progress. Compare I read the newspaper every morning, where the habit is stated plainly, with I am reading the newspaper right now, where the continuous form shows the action happening at this moment. The simple version is the plainer one to use, though choosing correctly between the two still takes practice.
Habits and general truths fit the simple present: Maya drinks coffee every morning. One stated routine, no extra helping words.
Finished actions fit the simple past: The train arrived late last night. It happened, it is done, and the verb says so on its own.
Future events fit the simple future: We will call you tomorrow. One act, one moment, with will carrying the time.
A worth-noting twist: I read every night before I sleep. Here read is present simple, a habit, even though the same spelling can also be a past form. The spelling of read is the same for present and past, and only the pronunciation changes, so the time reference and the pronunciation tell you whether it is the nightly habit or a finished action.
The name can mislead. Simple refers to the verb's plain form, not to how easy the rules feel. The simple present, for instance, adds an -s in the third person, so you say he works, not he work.
Of the three simple tenses, only one uses a helping word. The simple future is built with will, as in We will call you tomorrow. That helping word does only one job: it places the action in the future. The sentence still states the action plainly, the way We called you yesterday states a past action, so will marks the time without making the tense continuous.