Why does the computer only understand ones and zeros?
You don't understand, it works on that because in electronics there are only 2 possible on and off states 1 and 0 respectively.
So it all comes down to that. Like when you turn on the light in your house (1) or turn it off (0).
But Jacob, my computer understands decimals!!!! The question really is why do computers use a binary system to represent numbers.
The answer can be seen with this analogy: On the eve of an earthquake, which is better for remembering a number, using coins (with two states) or using dice (with six states)? Although it would take more coins than dice, the answer is obviously with coins because it is more difficult for them to change their state due to earthquake tremors than with dice.
In electronics the same thing happens and that is why computers use a system with components that can only be in one of two states, zero and one.