Network congestion is a scenario that occurs when the number of transactions on the blockchain network exceeds its capacity. Every blockchain has a maximum number of transactions that it can handle per second or TPS.
As a result, the amount of time that one node takes to process a transaction increases significantly. Typically, the primary cause of this overload is an increase in the number of people using the blockchain. Bitcoin and Ethereum network congestion motivates developers to develop more efficient blockchains and technologies, such as sharding.
https://learn.bybit.com/glossary/definition-network-congestion/