If the owner renting the apartment goes through a real estate broker, that broker charges some fee for their services (showing the apartment, etc.). In NYC, the fee is often paid by the incoming tenant. The fee, from what I've seen, ranges from 1 month's rent to 15% of annual rent (12 * .15 = 1.8 month's rent). If you're renting a $3000/month place with a 15% broker fee, you owe the broker $5400 up front when you sign the lease.
Brokers and sites that I've found claim you'll pay a broker one way or the other: if the owner pays the broker, then the owner increases the rent to cover the cost. If you find a "No fee" apartment (a designation that anyone who's rented in NYC knows, and it has a special status and banner on https://streeteasy.com/) then your rent is increased to match the market anyway.
Even if it's true that as a renter you pay for brokers no matter what, it's the deceit in advertising in NYC that irritates me. Brokers happily list "$3000/month" rent, and you have to email them to find out what their fee is. If you amortize the fee across a 12-month lease, that "$3000/month" might actually be $3250/month (1 month broker fee) or $3450/month (15% broker fee).