Wikipedia definition: is a slot or trench cut into the surface of a piece of machinable material, usually wood.
The above picture is some dadoes that I created for some drawers. I don't have a proper dado blade, so I will just run multiple passes on my table saw until I reach the desired thickness and depth.
Dado sets can be quite expensive so I haven't purchased one yet. I've read some tricks, where people will put multiple regular saw blades on their table saw side by side to remove more material. I may try this if I have a project that require a large dado. Below is an example and here is some info from www.woodgears.ca: