Gradients help display smooth transitions between two or more colors. Radial gradients in particular are defined by a center point from which the chosen colors radiate outwards in all directions.
Here are visual examples of radial gradient in action, and the corresponding code.
CSS
.gradient {width: 200px;height: 200px;background: radial-gradient(#d305f2, #4e025a);}
HTML
<div class="gradient"></div>