Monochrome photography is making pictures that don't have any color in them. Imagine that you have a big box of crayons with many different colors like red, green, blue, and yellow. Now, imagine that someone takes away all the colors but one. That means you only have one crayon to draw with. If you use only one crayon to make a picture, it will be a monochrome picture.
Monochrome photos only have shades of one color - usually black or white, but sometimes grey too. When you take a photograph, the camera sees the world in color. But if you put a black and white filter on your camera or change the settings, the camera only captures different shades of grey.
Sometimes, using black and white photography can make a photo look more interesting or express a certain feeling. It can help to remove distractions and draw the viewer's attention to other parts of the picture. It can also give a vintage or old-fashioned look to a modern photo.