logic of math's equations

[Otis]

what do you mean by "relative error" …

The word 'relative' is an adjective. The phrase 'relative error' tells us the amount of error is related to something.

Think about what you mean by "Y is regardless to Z".

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[Dr.Peterson]

what do you mean by "relative error" here I didn't understand that term

Relative error means the ratio of the error to the correct value. For instance, if the correct value is 121, but my answer is 123, then I am off by 2 (123 - 121), and my relative error is 2/121 = 0.0165 = 1.65%.

This is usually what matters most in approximation (though not always). It is part of what is meant when we say that something is negligible: one number is very small in proportion to another, so that when they are added, ignoring it will not result in a significant relative error.

If you can't discern the definition from what was said to you, why not search for something like "definition of relative error"? That's the way to learn! Presumably your goal is to learn, right? In fact, part of our goal is to help you learn how to learn, which is why we expect you to put in the effort to figure things out for yourself, rather than just tell you the answer.

By the way, if you had asked what "negligible" means, quoting your source, rather than using non-English terms like "Y is regardless to Z", this whole discussion might have been much more productive. People have been trying to teach you how to ask for help effectively, as well as how to learn, and this is an important point!