Variables are one of the most fundamental concepts in any programming language. A variable is container/holder to store/hold the data/information for future programming use or calculation purpose.

Custom properties sometimes also referred to as CSS variables or Cascading variables are nothing but an entities that contain specific values to be reused/accessed throughout a document and saves lots of time while editing large/huge websites.

CSS variables set/defined/declared using custom property notation (e.g., --base-color: black;) and are accessed/called using the var() function (e.g., background-color: var(--base-color);).