The plain-English definition
A cash-secured put combines two things: a short put option and enough cash to meet the purchase obligation if the option is assigned. The seller receives a premium and, in return, accepts an obligation to buy the underlying shares at the option's strike price before or at expiration.
“Cash-secured” describes the collateral, not the safety of the position. The reserved cash makes the obligation fundable, but it does not remove the risk that the shares could be worth much less than the strike price when assignment occurs.
How the position behaves
If the put expires out of the money, it generally expires without assignment and the seller keeps the premium. If it is exercised, the seller buys the shares at the strike price. Exercise can also happen before expiration, so the expiration date is not a guarantee about when assignment may occur.
The premium is the maximum gain from the put itself. The downside can be substantial because the acquired shares can continue falling in value. The conceptual breakeven at expiration is the strike price minus the premium received, before fees and taxes. This does not guarantee a “discount” to the market price that exists after assignment.
What changes after assignment
Assignment converts the option obligation into share ownership. From that point, the position behaves like owned shares: it participates in subsequent gains and losses, and the original option premium only provides a limited offset to the purchase cost. Closing the put before assignment instead ends the obligation at the option's then-current value.
Common misunderstanding
Keeping the premium is not the same as having a risk-free return. A small, fixed premium is exchanged for accepting a much larger possible decline in the underlying shares. Collateral answers “can the purchase be funded?”; it does not answer “will the purchase be profitable?”