This is an aircraft load factor graph, demonstrating how G loading or G-forces occur on the aircraft, and in turn, you when making level turns (level turns being holding a constant altitude and not climbing or descending. As we talk about being LEVEL in this TOPIC, what we mean is maintaining the same altitude). As you increase the bank angle of the airplane, the lift being generated by the wing is no longer just pushing straight down to keep the airplane in the air, this lift is now being directed at an angle which is ultimately what makes your airplane turn. The horizontal component of lift is what makes the aircraft turn.
We can break the total lift being generated by the wing into two separate “vectors” or forces; the horizontal component, and the vertical component. The vertical component is what keeps the airplane flying level, and as a result, the vertical component must remain the same to keep the airplane flying level regardless of turning or flying straight. Now to keep this vertical component the same when we are directing or lift off to the side by banking the airplane, we are going to have to increase our total lift on the wing, which in turn keeps the vertical lift vector constant, and increases the horizontal component of lift (making the airplane turn). This increase in the total lift (total lift in normal straight and level flight is about 1G force) is felt by the pilot and occupants of the aircraft as increased G-forces.
Ultimately, the more the wing tilts (banks), the more lift it must generate to keep the airplane in level flight. Obviously once the airplane banks to 90 degrees the amount of lift required becomes infinite, since the lift is only being directed sideways and no amount of lift from the wing (or pulling back on the controls by the pilot) could keep the airplane level.
If you want a few reference points, you should remember the load factor or G-forces in a 45-degree bank turn are 1.4 G’s, and the load factor in a 60-degree bank turn is 2 G’s.