What is the use of the HTML tag "canvas"?

The HTML <canvas> element is used to draw graphics, on the fly, via scripting (usually JavaScript).
The <canvas> element is only a container for graphics. You must use a script to actually draw the graphics.
Canvas has several methods for drawing paths, boxes, circles, text, and adding images.
Canvas Example
In HTML, a <canvas> element looks like this:
<canvas id="myCanvas" width="200" height="100"></canvas>