The gradient mesh tool in Illustrator is used to create organic blends between two or more colours. These can be used for simple backgrounds or to create the illusion of 3-dimensional plasticity in drawn objects. We show you how in our Adobe Illustrator courses. 
First create a rectangle with the Rectangle tool, and bring the Fill icon forward so that you’re working on the fill colour. In this example we’ve added as yellow base colour.
Then select the Gradient Mesh tool from your Toolbox. Click several times across the selected rectangle – say, four times diagonally across the shape. This adds anchor points which can be selected with the Direct Selection tool (the white arrow). Select one of these, then click on another colour in the Swatches panel – in this example we’ve chosen a red or orange swatch. Add several blotches of colour in this fashion. You can also drag the anchor points around to alter the shape of the colour blotch. In this way you can add depth and plasticity to forms created in Illustrator.
Whether you’re a complete beginner or self-taught, our courses will help you gain self-confidence in your Illustrator workflow, as well as adding to your professional skill-set. See many examples of our Illustrator clients’ work on our Facebook page.
Other related Illustrator Blog Posts:
Illustrator workspace
Illustrator basics
Illustrator clipping masks
Illustrator live trace
Illustrator brushes