I will try to explain a little about how work a confirmation within a blockchain, what's happend when you send or wait a transaction.
Blockchain,as his name say us is a group or chain of blocks linked to each other and is in this blocks where, among other things,contain ours transactions.
When a user sends any coin to some address, the transaction is not immediately inserted into the blockchain because it needs to be validated.
For a transaction are validated need be inserted into a block and at least one more block must be added to the blockchain.
The transaction is considered confirmed when it has been inserted into a block and counts with confirmations as the same number of subsequent blocks added to the blockchain, including the block into which it has been inserted.
Depending on the protocol used the validation will has different procedures, as far i know they can be "proof of Work" (PoW), "proof of Stake" (PoS), "delegated proof of stake" (DPoS), "proof of authority" (PoA), "proof of elapsed time" (POET).
For explain this I will use the way in which this process would be done with "Bitcoin", this ultra famous cryptocurrency is based on a "proof of work" protocol, in this case the proof of work is done by miners (all us seen news and images of huge swarms of computers "mining", well...they are the nodes we call "miners")...miners verify that a transaction is correct,if the signature corresponds to the sender's signature and what the fees that we pay to the miners.
When we try to make a transaction its not sent directly to the blockchain, instead its sent to a kind of "TEMPORARY DEPOSIT" usually called "MEMPOOL", in this place the transactions remain until they are inserted in a block to later be confirmed or rejected... these "mempools" sometimes receive huge amounts of transactions waiting to be validated and they tend to get congested at times of high activity on exchanges, etc.
Knowing that a block of Bitcoin blockchain cant contain more than 3,000 transactions and that an average of one is mined every 10 minutes, its logical imagine that the Bitcoin mempool is always full and very often contains more transactions than what's in a block and its strongly aggravated when there is a lot of activity on exchanges that leads to serious delays in confirmations (congested).
As a result of this huge amount of accumulated transactions in the "MEMPOOLS", miners are forced to choose which transactions to include and which to postpone,its to be expected that the transactions they will include or try to include in the first place will be those that contain the most high fees...this means that if fees set by the sender are too low, the transaction be the risk of not being selected to be included in the next block and also may be risk not even being included in the next block...this is the reason why sometimes many transactions wait even days to find a miner who decide agrees the transaction for be validated.
(Think of this as a taxi driver at an airport, he waits hours to have access to the passenger pick-up area (it entails not earning money during that time) when he is finally in the passenger pick-up area wishes his future passenger demanded a Long trip ... if he could know in advance where the passengers want to go (how much money he could earn) and if he could choose which passenger to choose, it would be great for him but not so much for us, right? Well, this is what miners do,choose which passenger he going attend).
Here is a small tip that maybe you did not know or maybe yes... the transactions that still remain in a "MEMPOOL" can be modified, for example to increase the fees and make it more attractive for miners,this in theory is possible as long as its inside a MEMPOOL but not all wallets allow you do this (not possible with almost any custodian wallets,third party wallets that hold your assets),for do this need a wallet that suggest to user what fees want to set for the transaction.
