what does linear do in css code example

Example 1: linear-gradient

background: linear-gradient(to left, #333, #333 50%, #eee 100%);

Example 2: html css background linear-gradient

background: linear-gradient(#333, #333 50%, #eee 100%);
\\code for a basic gradient background
   #grad {
  background-image: linear-gradient(red, yellow);
}