A CSS rule consists of a selector and a declaration block.
The selector in css indicates the HTML elements which you want to style and it could be any tag like <h1>, <p> and <h3> etc.
The declaration block in CSS can contain the one or more declarations separated by a semicolon(;). For the above example, there are two declarations:
Each declaration contains a property name and value and it was separated by a colon(:).
A property is a type of attribute of HTML element and it could be color, border and font-size etc.
The value are assigned to css properties which is applied to any HTML Elements. in the above example "green" is assigned to color property.
selector{property1: value1; property2: value2; ..........;}