It is the color you get when you mix any primary colors together.
It is very useful in interior decorating as it is subtle and stable to use, it is not dramatic like black or white, the other neutrals, therefore is is good to use with other colors as it does not dominate them.
An example of this is using hot colors, hot pink, hot blue, hot green.
These look funky and fun on their own but can be a little garish, with the addition of gray they become compromised and drawn back into line and make a statement but a more subtle one.
Gray is often used in industrial design, look the color of your keyboard, laptop, mobile, calculators, medical machinery, they are often gray. This means that they will suit any interior design
Gray is not technically a color, it is a neutral, but we will call it a color as it is easier to use in this context. or exterior painting color schemes as it is not offensive and combines well with a great deal of colors
Concrete is a very popular look at the moment and that is very gray, used for flooring, exterior walls, driveways and fences and teams up well with green landscaping
Gray (Grey) tinted glass can be fabricated, tempered, drilled and shaped like regular clear glass, but the color allows it to add an air of mystery and privacy to the space.
Gray is often a drab color, it goes by unnoticed unless it gets teamed up with a color of some substance like hot pink or brilliant orange.