Bitcoin transaction fees
Because a block on the bitcoin blockchain can only contain up to 1 MB of information, there is a limited number of transactions that can be included in any given block. During times of congestion, when a large number of users are sending funds, there can be more transactions awaiting confirmation than there is space in a block.
When a user decides to send funds and the transaction is broadcast, it initially goes into what is called the memory pool (mempool for short) before being included into a block. It is from this mempool that miners choose which transactions to include, prioritizing the ones with higher fees.
If the mempool is full, the fee market may turn into a competition: users will compete to get their transactions into the next block by including higher and higher fees. Eventually, the market will reach a maximum equilibrium fee that users are willing to pay and the miners will work through the entire mempool in order.
At this point, once traffic has decreased, the equilibrium fee will go back down.
Current network fee:
14 sat/byte - The lowest fee (in satoshis per byte) that will currently result in the fastest transaction confirmations (usually 0 to 1 block delay).
3 sat/byte - The lowest fee (in satoshis per byte) that will confirm transactions within half an hour (with 90% probability).
1 sat/byte - The lowest fee (in satoshis per byte) that will confirm transactions within an hour (with 90% probability).