Trend weight is a smoothed line through your daily weigh-ins that filters out random day-to-day swings and shows where your weight is really heading. It is the number you steer a bulk or cut by, not any single morning reading.
The reason is signal versus noise. Water, salt, carbs and gut content easily move your daily weight by one to two kilograms, while your real change per week is often just a few hundred grams. Without smoothing, that change disappears in the zigzag.
How it is calculated
The simplest version is a moving average over 7 or 14 days. Another common method is exponential smoothing as described by John Walker in The Hacker's Diet, which Lean is law uses by default: each new daily value counts for 10 percent, the previous trend for 90 percent.
Example: Your trend is at 80.0 kg and today the scale reads 81.5 kg. The new trend is 80.0 + 0.1 × 1.5 = 80.15 kg. A swing of 1.5 kg moves the trend by only 150 grams. This is a worked example.
What to keep in mind
Every kind of smoothing lags behind a real change, with the usual methods by about a week. While you are losing weight, the trend therefore tends to sit above today's reading, and below it while you are gaining. That is not a bug but the price of a calm line.
For decisions, look at the direction of the trend, such as the 7-day change or the weekly rate as a percentage of body weight. Judge any adjustment after two to three weeks at the earliest. And always compare weeks using the same method: switching methods changes the number on screen, not your body weight.