Comparables — "comps" — are nearby properties similar enough to yours that their recent prices say something about your own. Sales comps drive valuation: an appraiser or agent finds recent sales of similar size, age, condition, and location, then adjusts up or down for the differences. Rental comps work the same way for pricing a lease: what are similar units actually renting for right now, not what they are listed at.
The craft is in the adjustments and the honesty. A comp two blocks away in a different school zone, or a renovated unit compared against your original-condition one, can quietly skew the answer by a lot. Use several comps rather than one, prefer closed transactions over asking prices, and weight the most recent and most similar ones heaviest. For rent-setting, stale comps are the usual mistake — markets move, and last year's rents are not this year's.