Scalibility in terms of cryptocurrencies refers to the speed of the blockchain when confirming transactions. In the case of Bitcoin, scalability usually revolves around the 1 MB block size, which was implemented initially to limit spam attacks on the network, but now has created issues of network overload and delayed transactions due to its inability to deal with large volumes of transactions. Scalability can also refer to the need for ever greater processing power for the network to confirm all the incoming transactions.
Ethereum has also faced scalability issues due to a similar problem where all the transactions and smart contracts on the network must go through the Ethereum Virtual Machine (EVM). There are solutions to these scalability issues in development, such as the Lightening Network for Bitcoin and the Raiden Network for Ethereum. There have also been other solutions such as SegWit for Bitcoin and sharding in Ethereum, but scalability remains one of the largest hurdles faced by blockchain technology when it comes to mass adoption.