Blocking in refers to an initial painting process of blocking in the general colors and shapes on your canvas. This is a starting procedure used mostly for oil painting.
The purpose of blocking in is to lay down the general composition and color harmony without having to worry about the tedious details. You can really get a feel for where the painting is going with this starting procedure. Generally, very large brushes are used for this and the paint is thinned.
The end result of blocking in will be a no frills painting - what you would see if you squint. From there, you can start to add details and make any adjustments.
For blocking in you will usually start with toned down versions of the main color you are blocking in. You can then build up the saturation of the color as needed later in the painting. If you block in the colors with too much saturation the painting could be overwhelming and you would spend the rest of your painting time trying to tone it down.
Much of the blocked in areas are left exposed in the final painting.
Blocking in is a very popular technique for plein air painting as it allows you to cover the canvas very quickly with the general layout and color composition. You can then aim to capture the fading details of the environment.
(If you want to learn more about color mixing and painting in general, I invite you to join my free email course, 7 Days to Better Paintings).