A gas limit caps the quantity of gas available for a transaction or block. Gas measures protocol-defined resource consumption; the limit is a quantity, while gas price determines what each unit costs.
A transaction's budget
A transaction specifies a gas budget large enough for its intended operation. An insufficient limit can cause rejection before inclusion or an out-of-gas failure during execution. In the latter case, execution changes revert, but consumed gas still costs money.
Unused gas is not charged merely because it was included in the limit. For example, if the limit is 100,000 and the receipt records 60,000 gas used at an effective price of 20 gwei, the execution fee is 0.0012 ETH. The remaining 40,000 gas is unused.
The block's capacity
The block gas limit constrains aggregate gas consumption across its transactions. It is separate from each transaction's budget. Ethereum's Fusaka upgrade introduced a maximum declared transaction gas limit of 16,777,216, independent of the block limit. Declaring more makes a transaction invalid even if expected usage is lower.
Limit versus priority
Raising a gas limit does not itself raise the price offered to validators or guarantee faster inclusion. Wallet estimates help size the budget, but execution requirements and network rules still matter.