Market Maker vs Market Taker
A maker places an order that waits in the book; a taker fills an existing order immediately.
In depth
Makers provide liquidity, so exchanges usually charge them lower fees (or pay rebates). Takers consume liquidity and pay a bit more for instant execution. A limit order that rests is a maker order; a market order is always a taker.
Why it matters
StableX’s fee schedule lists maker and taker rates separately — this is why.