Definition
Chip select (often written CS, or SS for slave select) is a control line that tells a single chip on a shared bus, "this message is for you." When several peripherals sit on the same data and clock lines, chip select is what keeps them from all responding at once. It is a small detail that turns a point-to-point link into a manageable multi-device bus.
How it works
On a bus such as SPI, the clock and data lines are shared by every peripheral, but each peripheral gets its own chip-select line back to the controller. The line is usually active-low, meaning the controller pulls it down to wake a chip and lets it float high to ignore the chip. To talk to a particular flash or sensor, the controller asserts that one device's chip select, clocks the transaction, then releases it. Any chip whose select line is idle keeps its data output in a high-impedance state, electrically stepping aside so it does not interfere with whoever is active.
Why it matters in practice
The chip-select scheme is why SPI scales in speed but costs a pin per device: every extra peripheral needs another select line and another free pin on the controller. When you read a flash chip with an external programmer during a repair, holding chip select correctly is part of what makes the read succeed; a floating or mis-wired select line is a common reason a dump comes back as garbage. On boards where pins are scarce, designers sometimes add a small decoder so a few control lines can drive many chip selects.
For the bus this line most often controls, and the rest of its signal set, see SPI.
In Simple Terms
Chip select (often written CS, or SS for slave select) is a control line that tells a single chip on a shared bus, “this message…