Through the middle of Guyana runs the Essequibo River, the largest river between the Orinoco and the Amazon. Venezuela has historical claims to control the territory to the Essequibo. In fact, it was originally Spain that claimed the territory, against the Netherlands and Britain, as Guyana was formerly a British colony. De facto the area has been controlled by Britain then independent Guyana since the early 1900s.
But this has not stopped Venezuela from attempting to press its claim, resulting in a 1966 Geneva Agreement (which Venezuela interprets as merely promising to attempt to reach an agreement), multiple Venezuelan military actions, and economic sanctions against countries that attempt to help Guyana develop the area.
