A CSS comprises of style rules that are interpreted by the browser and then applied to the corresponding elements in your document. A style rule is made of three parts:

Selector: A selector is an HTML tag at which a style will be applied. This could be any tag like or etc.

Property: A property is a type of attribute of HTML tag. Put simply, all the HTML attributes are converted into CSS properties. They could be color, border, etc.

Value: Values are assigned to properties. For example, color property can have the value either red or #F1F1F1 etc.

Syntax– selector { property: value }

Example: You can define a table border as follows:
table{ border :2px solid #C00; }

Here table is a selector and border is a property and the given value 2px solid #C00 is the value of that property.