Price Slippage is the change in token price caused by the total movement of the market. Price Slippage is reflected as the difference between the price you expect to receive after swapping vs what you actually receive after the swap is complete.
The minimum amount you receive from a trade is determined by the market price and your slippage limit setting. When you are using the Uniswap web app, you will receive the market price that is offered based on the slippage limits. The Uniswap web app sets a default slippage of 0.50%.
There are two factors that contribute to Price Slippage:
Some tokens and token pairs are not traded as much (very new, not popular, etc) and have low liquidity. Due to the low demand of these tokens, when a swap is initiated the price may dramatically change because the difference between the lowest asking price and the highest bid is very high.
When a token price is volatile, it means that is can change rapidly or unexpectedly. This can cause a dramatic change in price that effects the market for that token price.
The example below shows how the Uniswap web app informs you of price slippage:
Here, the expected amount of DAI that you are going to receive is 9.16624.
The minimum that you can receive in this transaction is be 9.12064, which is 0.50% less than your expected price.
You can more learn about adjusting your slippage at your own risk using the Expert Mode feature.