What It Means
A market order says "fill me right now" at whatever the best available price is — it guarantees execution but not the price. A limit order says "fill me only at this price or better" — it guarantees the price but not that the trade happens at all.
How It Works
A market order matches immediately against the best resting orders on the other side of the book, however many price levels it takes. A limit order sits on the book at your specified price and waits, only executing if the market actually trades there.
Example
A stock's last traded price is ₹500, but it's thinly traded. A market buy order for a large quantity might fill across several price levels, averaging ₹504 instead of ₹500 — that gap is slippage. A limit order set at ₹500 avoids that, but if the price never comes back down to ₹500, it simply never fills.
Warning
Using a market order on a low-liquidity stock is the single most common way traders get a worse fill than expected — check the bid-ask spread before choosing market over limit.