Gas
The unit that measures how much computing work a transaction uses, and therefore what it costs to run.
Every operation has a fixed price in gas, with writes to storage among the most expensive because they enlarge the data every node must keep forever. The fee a sender pays is gas used multiplied by the price per gas, and the gas limit the sender sets is a ceiling on how much work they will pay for; if execution exceeds it the transaction fails, its changes are discarded, and the gas already burned through is still charged. A block also has a gas limit, and that cap is what makes space scarce and creates a fee market. Readers meet gas in wallet confirmation screens, in failed transactions marked out of gas, and in contract design where developers optimize storage use.
In practice
A plain transfer of ether costs a fixed 21,000 gas, while a contract call that writes new storage slots costs considerably more.
The common misunderstanding
Raising your gas limit does not make a transaction faster or more likely to be included; the tip affects ordering, while the limit only sets the ceiling on work.