how to put background or solid block of color on a video
I've not used Window Movie Maker, so I don't know it's abilities. However, the area where the text shows up is called a "lower third" because it's in the lower 3rd of the screen. If Movie Maker has the ability to add titles, it might have some named something like "Lower 3rd".

If it doesn't have such a title option already, you can easily create something like the one shown in the video above. Simply make an image in your favorite image editor that has the color bar you want in the lower 3rd area, and is transparent everywhere else. You'll need to save it with transparency, so a format like .png or .tif is best. Formats like .jpg don't save transparency.

Once you have the color bar with transparency around it, you can layer it over your video. (I assume Movie Maker has some way to layer video?) Then put your text over the color bar. Or, if you don't have that ability, you can bake the text into it in your image editing app.