Ok first understand that ATP is the ONLY energy source that your muscles can use for energy.
YES glucose (carbs) and fat can be used to make energy, but only by being used to produce ATP.
Your capacity to store more ATP is limited and you can't really increase it significantly. BUT anytime you increase the efficiency of your energy systems you can produce more ATP faster.
Now, how does creatine affects ATP?
By giving one of it's phosphate molecule.
Here's what happens.
ATP produces energy by breaking off one of its phosphate molecules. ATP = one molecule of adenosine and 3 molecules of phosphate.
Breaking the chemical bond of one of the phosphates liberate energy.
ATP = ADP + energy + P
Creatine is stored in the muscles as creatine phosphate (one molecule of creatine and one molecule of phosphate).
When doing intense muscular work the creatine will give it's phosphate to ADP to make it into ATP again so that it can be used to produce more energy
Normally you have enough ATP stored in a muscle for about 3-5 seconds of INTENSE work (your body will not rely on muscle ATP stores to do low intensity work). Creatine can extend that to 9-10 seconds and creatine supplementation can boost it up maybe to 12-15 seconds.
Not a huge difference, but it an make a 1 or 2 reps difference in a set.
I personally am NOT a big fan of creatine but I have nothing against people using it.