What is difference in UniswapV2 router address and pair address? and when they occur?

Question

I am currently working on a pre-built ERC20 contract code that uses two different addresses from UniswapV2 to do validations in its logic.

uniswapV2Router = IUniswapV2Router02(0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D);

uniswapV2Pair = IUniswapV2Factory(uniswapV2Router.factory()).createPair(address(this), uniswapV2Router.WETH());

Inside the _transfer function it checks if from or to parameters equal to address(uniswapV2Router) or uniswapV2Pair.

I am trying to understand why and when to use each of those addresses especially uniswapV2Pair and when my transfer function is called with any of those addresses as input parameters.