Scaling
Scaling refers to adjusting the functionality of software, such as a blockchain, in order to account for more people using it. Bitcoin, for example, was not meant for widespread use; as it has grown in popularity, its use has become less convenient. For context, imagine if every time you wanted to make a purchase using a credit card, you had to wait 13 minutes or more for the payment to go through. Obviously, this would affect the viability of credit cards as a method of payment. This is why scalability is a major issue with popular cryptocurrencies - some blockchains were unintentionally designed in such a way that the more people used them, the more time it took to verify and execute a cryptocurrency transaction. Increased traffic on the network also affects how much a typical user needs to pay in mining fees to verify the transaction on the blockchain in a viable amount of time. In other words, if you don't want to wait 13 minutes for a transaction to go through, you may have to pay more in fees to expedite the transaction.[1]
Certain solutions, such as bitcoin's Lightning Network and Ethereum's Liquidity.Network project have been created in order to address this problem.[2]