The word "monotonous" means "boring". Monotonous things are boring because they continue for a long time without changing. For example, a song that has the same chorus repeated 30 times without changing could be called "monotonous":

This song is so damn monotonous!

Some examples of things that you can describe as "monotonous" include:

someone's monotonous voice

a monotonous landscape

a monotonous task