The basics
What ARV is, and why your offer starts here
The after repair value (ARV) is an estimate of what a distressed house will be worth after an investor makes all the needed repairs. It's not what the house is worth today, and it's not what the seller wants. It's what a fixed-up version of it would sell for.
Every number after this one depends on it. Your maximum allowable offer starts with ARV, and so does your buyer's profit. Get the ARV wrong by a little and the whole deal can fall apart at closing.
Agents do this with a CMA (comparative market analysis), which most people call pulling comps. You don't need MLS access to get a solid number. Public sold data on the big listing sites is good enough to start, as long as you pick the comps carefully.
Subject propertyThe house you're thinking of putting under contract.
CompsComparable homes nearby that sold recently. For ARV, you want the renovated ones.
Price per sq ftSold price divided by living area. It lets you compare houses of different sizes.
AVMAn automated valuation model, like a Zestimate. A starting point, never your ARV.