The speaker describes how he walked around and felt as lonely as a cloud. He doesn’t say, "walked around," but uses the much more descriptive word "wandered."
"Wandered" means roaming around without a purpose, like when you explore something. So it’s not necessarily a bad thing. But in its metaphorical use, "wandered" can mean feeling purposeless and directionless in general. As in, you have questions like, "What’s the meaning of my life?"
The first concept that we want to take a look at is that the cloud is "lonely." Asking questions about what this means will help us get into the poem.
Are clouds lonely? Well, maybe the ones that float about valleys ("vales") and hills are lonely. It's more likely, the speaker is projecting his own loneliness on the clouds. But that still doesn’t explain the strange image, because clouds usually travel in groups. (Except in cartoons where you can have a single rain cloud following Wiley E. Coyote around just to ruin his day.)
Maybe a cloud is lonely because it is so far above the rest of the world. Its thoughts are just so "lofty," and maybe the speaker’s thoughts are, too.
Also, the cloud could be lonely because it floats over a natural landscape with no people in it. Maybe the speaker has thought of hills and valleys because he happens to be "wandering" through such a landscape.
These are some of the questions we’re hoping the poem will help us sort out after this mysterious beginning.