Definition of a limit using epsilon delta

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I am a little confused regarding the definition of a limit using $\varepsilon$,$\delta$.First of all I would like to know if the full understanding of the limit idea requires any prior knowledge about $\varepsilon ,\delta$ or is it a notion that first appears in the definition of limits.Second I am confused about what epsilon depicts. Does it depict the total of all positive real numbers? In addition to that why is $\delta $ expressed as a function of $\varepsilon$ ?