What is supply and demand?
In short
Supply is how much of something sellers offer. Demand is how much buyers want. Together they are the usual explanation of why a price settles where it does.
In plain words
If buyers want more than is available at the current price, sellers can often charge more, and a higher price draws out more supply or puts some buyers off. If goods are piling up unsold, the price tends to fall. This is a sketch of competitive markets, not a law that every price obeys. Rents, wages, and rail fares are also shaped by contracts, rules, and how much power one side has.
A simple example
A wet week in a seaside town. Shops have a limited stock of umbrellas, and visitors suddenly want them. Until new stock arrives, the scarce umbrellas go to people willing to pay more, or they simply sell out. On a dry week the same umbrellas may be discounted. The weather shifted demand. The stock limited supply.
Why it matters
The same picture helps with bigger questions: why fruit is cheaper in season, why a popular concert costs more than an empty midweek film, and why a shortage of workers in one trade can lift wages there. Policy still matters. A price cap, a tax, or a monopoly can stop the price from doing that job, sometimes on purpose.
Easy to mix up
Demand is not the same as need. People need water and they need shelter; what they buy still depends on price and income. Buying more because the price fell is not the same thing as wanting more even at the old price. And supply is not just the stock in a cupboard. It includes how much producers are willing to make at a given price.