What happens during Glycolysis?
Glycolysis is a process of which converts Glucose(Described as C$_{6}$ in your diagram), to two Pyruvate (2C$_{3}$). Then in downstream of glycolysis, Pyruvates are then converted to AcetylCoA to be used for many things in body. Glycolysis is part of a metabolic pathway, and in total it uses 1 Glucose, 2 ATP, 2 NAD+ to produce 2 Pyruvate, 4 ATP and 2 NADH. So those curve lines represents the Total input and output of energy. Each step is facilitated by different enzymes, so you could search for enzymes in each step for more information about each reaction. Wikipedia has table of each reactions.