In a linear gradient, the colors flow in a single direction, for example from left to right, top to bottom, or any angle you choose.
Syntax
To create a linear gradient you must define at least two color stops. They are the colors the transitions are created among. It is declared on either the background or background-image properties.
background: linear-gradient(direction, colour-stop1, colour-stop2, ...);If no direction is specified, the default transition is top to bottom.
Examples
Top to bottom:
background: linear-gradient(red, yellow);
Left To right:
To make it left to right, you add an additional parameter at the beginning of the linear-gradient() starting with the word to which indicates the direction:
background: linear-gradient(to right, red , yellow);
Diagonal gradients:
You can also transition a gradient diagonally by specifying the horizontal and vertical starting positions, for example, top-left, or bottom-right.
Here’s a sample for a gradient starting from the top-left:
background: linear-gradient(to bottom right, red, yellow);
Using angles to specify the direction of the gradient
You can also use angles, to be more accurate in specifying the direction of the gradient:
background: linear-gradient(angle, colour-stop1, colour-stop2);The angle is specified as an angle between a horizontal line and the gradient line.
background: linear-gradient(90deg, red, yellow);
Using more than two colors
You’re not limited to just two colors – you can use as many comma separated colors as you want.
background: linear-gradient(red, yellow, green); 
You can also use other color syntaxes like RGB or hex codes to specify the colors.
Hard color stops
You can not only use gradients to transition with fading colors, but you can also use it to change from one solid color to another solid color instantly:
background: linear-gradient(to right,red 15%, yellow 15%);