Scale weight is the number your scale shows on a given day: your raw reading, including every random influence. If you weigh yourself several times on one day, the weight trend calculator averages them into a single daily value.
Scale weight contains two parts: what is really changing and what happens to be added on that day. Water, salt, glycogen and gut content easily move the reading by one to two kilograms. Your fat, on the other hand, changes slowly, often by only a few hundred grams per week.
Example: On Monday the scale reads 84.1 kg, on Tuesday 85.6 kg, although you did nothing differently. A trend weight of 84.2 kg this week versus 84.6 kg the week before is still real progress of 400 grams. These numbers are an example.
How to handle scale weight
Look at the number, but do not judge it. It is allowed to be high. Decisions about calories or training are made on your trend weight and its change per week, not on a single Tuesday morning.
Still log every reading, including the unexpectedly high ones. If you only record the good days, you get a nice curve and a wrong one. Ignoring a reading is meant for genuine measurement errors, for example when you accidentally weighed yourself with clothes on. Weighing under the same conditions does not make scale weight calm, but it makes it comparable.