Rental yield expresses a property’s rent as a percentage of what it costs. Gross rental yield divides annual rent by the property price: $24,000 of rent on a $400,000 property is a 6% gross yield. Net rental yield goes further, subtracting operating expenses from the rent before dividing, so it reflects what the property actually nets.
Yield is close kin to cap rate; the practical difference is that yield is often measured against the price you paid, while cap rate is usually taken against current value, and “gross” versus “net” changes the answer a lot. Whichever you use, be clear about which version you’re quoting so you’re comparing like with like.