In Simple Words
When you send bitcoin, your transaction is broadcast to the network. At first, it may appear as unconfirmed because it has not yet been included in a block.
Once the transaction is included in a valid block, it receives its first confirmation (1 confirmation).
When another block is added on top of that block, the transaction has 2 confirmations. The next block brings it to 3, and the count keeps increasing in the same way.
This does not mean the same transaction gets checked again and again. Nodes validate blocks, while the confirmation count tells you how deep your transaction is in the chain.
As more blocks are built on top, replacing that part of the chain becomes increasingly difficult. That is why some wallets and exchanges wait for additional confirmations before treating a payment as final.