"Endeavor" as a verb that takes an object means something very similar to the verb "to try." For example, "He endeavored to change the law, but was ultimately unsuccessful."
"Endeavor" as a verb without an object means something like "to strive," or to give a lot of effort. For example, "She must endeavor if she wants to improve her skills." This form is not very common in American English.
"Endeavor" as a noun just means a hearty effort. "Through his tireless endeavors, the restaurant was a big success."