Each transaction has a size (in bytes) and a block has a limited size.

When you make a transaction you define (or your wallet defines) the fee per byte you are willing to pay. Your transaction will be broadcast to all nodes and will be waiting in the MEMPOOL to enter in the next block.

However, the mempool has unlimited size while a block has a limited size of 1 mb. So, transactions from the mempool who pays a higher fee per byte will go to the next block first.

For example, if you pay 2 satoshis per byte (2 sat/byte) and there are thousands of transactions of 150 sat/byte, you won't get into the next block.

Every 10 minutes or so a block is mined and some transactions from the mempool get into that block. The miner who mines it chooses the best paying transactions (as he collects the fees) and the cheaper ones will be waiting for the next block.

Once your transaction gets into a block it get its first confirmation. When the next block comes your transaction get 2 confirmations. Confirmation is the number of blocks subsequent to the block it was inserted into.