The Illustrator mesh tool can be found in the toolbar
Select a path and select the icon and click in the middle of the path. You will see a point added to the center of the path and it will be using the current color. While this point is selected, you can also go to the fill color in the panel and change the color or use the swatches panel. Set it to red, green etc. If you hover over anywhere on the path, the cursor will display a + and you can click and add yet another point. Again, this will be using the current color. As you add them, you will see the color spread throughout the document based on the points and the area. You can select the point and drag to another position and as you do so, the colors will re-align based on the new position.
You can select and move the points but you can also manipulate the direction points / handles as well. You can drag them out or in and change the angle and this will create creases or folds in the artwork and colors.
You can select a point and press the delete key and that will remove the current selected node.
You can use the points to warp the path as well as add color to the design. Go to the edge of the path and select a point or add a point and drag away from the original curve and this will distort the path and colors.
You can use that (also found in the panel next to the selection) to manipulate the points as well as the area in between the points and you can drag in and out the artwork in between the points. You can also select multiple areas with this as well as multiple points and the result will be an abstract shape design with multiple folds of color. You can use it to manipulate the direction handles.
You can use the feature to add streaks of color by creating very narrow areas between the points.
Use the direct selection to select a number of points, perhaps all the points in the object or objects. Go to one of the warping (scallop, crystallize etc) and apply to the current selected lines and points and you will the lines are warped as well as the colors. You can use this to create all kinds of painted like effects with your object. You can add live effects to individual parts of the design, so you can't blur parts of it or add transformations.
You can duplicate the path with the points and then use the feature to change the color of the duplicated item. You can select the entire path and then change the blend mode to darken or difference and blend between the two or more artworks.
You can change the opacity for the entire path but you can also use the feature with points and change the color but also change the opacity via the control bar. When you select a point or points and go to the opacity entry and set it from 100% to 20% you will then see some transparency in your design. Each point can have a different opacity and as you drag and change the position of the points, the opacity will also change and spread in different ways.