What is narrow gauge?   Technically, narrow gauge is the name given to any railway where the running rails are set closer together than the 'standard gauge' of 4' 8½". However this does over simply what is actually considered as narrow gauge.
Railways with gauges of around 3'6" gauge and over are built very much along standard gauge lines, rather than having the 'charm' associated with the smaller gauges, and so many people choose to think of these as something separate from narrow gauge.
Also lines of 15"-18" or below (again people do not agree on a definition) are often considered miniature, and again excluded from being thought of as narrow gauge.

What is 009?   This is the name given to one of the popular scale/gauge combinations used to model narrow gauge railways. The name literally mean 'OO' scale on 9 millimetre gauge track.
Assuming you do not wish to do everything the hard way then the trick with narrow gauge modelling is to select a commercially supported scale that results in track of the correct gauge also being available and commercially supported. This works with OO9, as when using OO scale(4mm:1 foot) N gauge track (9mm gauge) scales out to 2' 3". This combinations means that scenic and non railway items can be selected from a wide range of 00 scale commercial products, whilst the required chassis are available from the N gauge manafacturers.
This scale gauge combination is frequently used to represent gauges between 2' 0" and 2' 6".

This table lists other combinations that are close to 009 in either scale or gauge
Namemm/footGauge(mm)Description
00n12412Used to model 3' gauge prototypes
HOe3.59European verison of 009, but using HO scale to represent 2' 6" gauge.
HOm3.512Used in Europe to model metre gauge prototypes.
HOn303.59Scales out to 2' 6" gauge, popular in US
HOn33.510.5Used to model 3' gauge, mainly in US
HOm3.512Used in Europe to model metre gauge prototypes.
Sn24.89 or 10.5Used for 2' gauge prototypes