In short: accuracy tells you how close your current portfolio is to your chosen target portfolio.
The longer version: When you sign up for Passiv, you create a target portfolio that you want your account to follow. For example, let’s say you want to buy four funds and you want them each to be worth 25% of your total portfolio. Before you actually buy those funds, your investment account has nothing but cash in it, so the accuracy is 0%. When you buy the funds, you buy enough of each so that each one is worth exactly 25% of your portfolio. At this point, your accuracy is 100% because your holdings perfectly match your target.
However, over time, different funds have different returns and this throws your portfolio off your target allocation. Maybe one fund is now worth 30% of your portfolio and another is worth just 20%, while the other two stayed at 25% each. When this happens, your accuracy percentage will be lower and Passiv will suggest trades to bring your portfolio back in line with your target. By default, Passiv is set to Buy-Only – to see why, read below.