Neutral is the name given to the various interactions between players when both are in a neutral state. This is a very broad topic with lots of gray area, so no article on the topic can be comprehensive. See the External Resources section for a few guides by others that discuss the topic further.
The goal of neutral is to get into obtain a positional advantage while stopping the opponent from doing the same.
Each character has a desired position on the screen in relation to the opponent. The advantageous position may change depending on what resources they have available to them (super meter, burst, character specifics, etc).
Moving to an Advantageous Position
Exploiting an Advantageous Position