The Krebs cycle is a sequence of chemical reactions involved in cellular respiration, a process by which food molecules are broken down to get chemical energy. It takes place in the mitochondria.
It starts with a pyruvate, a chemical compound that has three carbon atoms. A pyruvate is a result from Glycolysis, therefore it is related to glucose as Glycolysis is the process in which glucose is broken into two things: energy and the pyruvate.
So, the Krebs cycle starts with this pyruvate and, at the end of it, the glucose is completely broken down and all of its carbons are separated and combined with oxygen to form carbon dioxide.
The chemical bonds releases energy, that is stored in energy-carrier molecules. Those molecules are 10 NADH (but 2 are from glycolysis), 2 FADH2 and 4 ATP (but 2 are from glycolysis too). NADH and FADH2 are high energy electron carriers, used to transport electrons that comes from Glycolysis and from the Krebs Cycle. An ATP is an energy-carrying molecule.
In that way, energy in form of ATP is released at the end of the process.