Why is the network congested? I'm reading a lot of people saying that is taking around 3/4 days to confirm a transaction.

What are the reasons for this being happening?
The Bitcoin network confirms one block of transactions approximately every ten minutes. The blocks are produced in a random process by miners. Each miner may select their own set of transactions to confirm, and therefore whoever is successful at finding the next block determines which transactions get confirmed.

Since the blocksize is limited to 1,000,000 bytes, the number of transactions that can be confirmed per block is limited as well. Miners maximize their mining revenue by selecting the transactions first that pay the largest fee rate, i.e. the greatest amount of satoshis per byte of blocksize the transaction requires.

Thus, users are bidding with "transaction fees" for inclusion into a limited supply of blockspace. The network is "congested" when a lot of more transactions bid for inclusion than can find space in the blocks.

Multiple proposals to upgrade the network's capacity exist. Especially the segregated witness proposal has been production ready for several months and is waiting to be activated. Unfortunately, an insufficient portion of the hashrate is signaling readiness for its activation at this point in time.