Both "a lot" and "a lot of" mean "a large amount," "many" or "several." The main difference is that you use "a lot" at the end of a sentence as an adverb, and it cannot be followed by a noun.
One good example: "I enjoy swimming a lot."
"A lot of," however, can be followed by a noun, and you use it in the middle of a sentence!
Example- "We need a lot of help with this project."
Hope this helps. ^^