Cellular respiration is the process your cells undertake to derive energy from carbohydrates. Carbohydrates are broken down by enzymes to the simplest sugar, glucose, a 6C sugar. There's a lot of stored energy in glucose, and in order to get it out, the cell has to convert glucose to a form of energy it can actually use.
There are three primary steps: glycolysis, the Krebs cycle, and the electron transport chain. Together, these steps yield 30-32 ATP (a molecule that contains energy).
In glycolysis, glucose is converted via several reactions to pyruvate (sometimes called pyruvic acid). This produces 2 ATP (4 ATP produced total - 2 ATP used to break down glucose)
Pyruvate is then funneled to the Krebs cycle. The main point of the Krebs cycle is to convert pyruvate into the high energy intermediates ATP, NADH and FADH2. NADH and FADH2 are similar to ATP, but just energy in another form. However, the cell prefers to only use ATP, so these have to be further converted.
NADH and FADH2 are funneled into the electron transport chain. They still contain a lot of energy. If all this is energy is released at once, it can't be harnessed. So, NADH and FADH2 slowly release their energy in the electron transport chain. The energy they release is used by the ETC to pump hydrogen ions (protons) against their gradient. These protons then go through the enzyme ATP synthase, which ultimately converts the energy that used to be NADH and FADH2 to ATP.