Body composition describes how your body weight is split between fat mass and fat-free mass: fat on one side, muscle, organs, bone and water on the other. Two people of the same weight and height can be composed completely differently.
Why the scale cannot show it
The scale only measures the total. It does not tell you whether you are losing or gaining fat or muscle. During a recomp, building muscle while losing fat, your composition can change considerably while your weight stays almost the same.
Example: In a study of 48 resistance-trained men and women (Antonio et al. 2015), the higher-protein group gained an average of 1.5 kg of fat-free mass and lost 1.7 kg of fat over eight weeks. Their body weight changed by just 0.1 kg. Looking only at the scale, you would have seen a standstill.
How to track it
Add numbers to your trend weight that say something about composition. The easiest is waist circumference: if your waist grows much faster than your weight during a bulk, you are probably gaining too fast. Add photos taken under the same conditions and your strength on the main lifts. A body fat estimate, for example with the Navy method, can round this out but is often too imprecise for small changes.
How your composition shifts when you gain or lose weight also depends on your starting body fat (Forbes 2000). That is why body fat percentage is the key number when deciding whether to start with a bulk or a cut.