A maker is an order that rests on the book instead of executing immediately, adding liquidity that someone else can trade against. Makers choose their price and wait for a counterparty, which means better prices than crossing the spread but no guarantee of a fill.
Making is the structural position of a market maker: quote both sides, earn the spread when both fill, and manage the inventory you accumulate when only one side does.
The risk that defines making is adverse selection — the fills you get are disproportionately the ones you did not want, because the trader crossing into your quote often knows something you do not, or is simply faster.
Estimating market-making revenue as spread times volume. Realised profit is consistently far below that, because the fills you get are selected against you — see adverse selection.
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