Here's why:

* Structure: The nitrate ion has a central nitrogen atom bonded to two oxygen atoms and a single negative charge.

* Resonance: The negative charge is delocalized over all three atoms, meaning it isn't fixed on any one oxygen. This is represented by drawing multiple resonance structures.

* Electron Movement: The double bond can shift between the nitrogen and each of the oxygen atoms, resulting in three different possible structures.

Here's a simple way to visualize it:

1. Structure 1: Double bond between nitrogen and one oxygen, single bond between nitrogen and the other oxygen.

2. Structure 2: Double bond between nitrogen and the other oxygen, single bond between nitrogen and the first oxygen.

3. Structure 3: A formal single bond to each oxygen and a positive charge on the nitrogen and a negative charge on each oxygen.

These three structures are equivalent, and the actual structure of the nitrate ion is a hybrid of all three.