'Negative gearing' is one of the crazy tax shenanigans involving real estate. Here's an ELI5 about it.

Imagine you earn 100,000 a year salary. You also have an investment property that earns 20,000 a year rent. You have expenses for that property though, repairs, agents, fees, and most importantly interest on the mortgage.

If your expenses are 5,000 for the property, your property earns you 15,000 net, and you pay tax on the 15,000 (plus your unrelated 100k salary) This is a positively-geared property. It makes you profit, you pay tax on it. It's a profitable investment.

If your expenses for the property are 20,000, then you pay no tax on the income from that property, every cent is spent on the property.

If the property costs you MORE than 20,000 in costs though (let's say 30,000), you are making a 10,000 loss on the property. Not only do you pay no tax on the income, the loss you make is deducted from your 100k salary. If your costs are 30,000k a year, you pay no tax on the rental income, and now only have to pay tax on 90k of your 100k income. This is called negative gearing.