For example, take a look at this phrase: We know someone or something went to the movies. But we don't know who or what went. So this phrase is a fragment. Orders and commands (like Go clean your room! or Stand up straight!) might look like fragments, since they don't have subjects you can see.

A sentence fragment is a group of words punctuated as a sentence, even though it’s not a sentence. Examples: Dr Johnson ate a big meal. And went to work afterward. There is a remedy for everything. Except death. A word of caution though. Don’t take his words as gospel truth.