Equity is the difference between what your property is worth and what you owe on it. If your home is worth $900,000 and your loan balance is $550,000, you have $350,000 in equity.
But not all of that equity is accessible. Lenders typically allow you to borrow against your property up to 80% of its value (without paying LMI). The portion above your current loan balance and up to 80% of the property value is called your usable equity.
This $170,000 could be used as a deposit for an investment property - without needing to save additional cash.