A block bundles transactions taken from the mempool, stamps them with a cryptographic-hash of the previous block, and appends the bundle to the chain. Once it is published, every full-node checks the rules independently and either accepts or rejects it.
Blocks have finite room, measured in bytes or gas units. When demand exceeds that room, transactions compete on fees and the cheapest ones wait. That is why network fees spike during a hot mint or a violent price move.
Example: a Bitcoin block holds roughly 1-2 MB of data, or a few thousand transactions, every ten minutes on average. If 15,000 transactions are queued, at least three or four blocks must pass before the queue clears, assuming no new ones arrive.
Related: blockchain, block-time, mempool, blockspace