There are often two ways of interpreting the demand and supply curves.
The demand curve consists of all buyers -- some who value the good more than the market price, and some who value it less. For more quantity to be produced, you have to lower the price so that those who value it less will buy. It makes sense that it is downward sloping.
Similarly the supply curve consists of all producers -- some who can produce the good at a lower cost than the market price, and some who can't. In this case, for more quantity to be produced, you have to raise the price so that those less efficient firms will be motivated to produce. It makes sense that the curve is upward sloping.
Marginal benefits and costs
Now we look at the curves in the context of one producer and one consumer. The demand curve of the consumer is downward sloping because of marginal utility. This makes sense to me.
What doesn't make sense to me is that the supply curve is upward sloping because marginal costs increase. Why is it that it costs more to make the next toy than it did the last?
If I've had any misconceptions or misunderstandings, please point them out to me. Thank you for the help!