To add weight you eat above your maintenance calories (TDEE). For lean muscle rather than fat, the surplus should be modest — enough to fuel growth, not so much you just store it.
The GAIN figure above adds a controlled surplus over your maintenance calories. A small surplus (roughly 250–500 kcal/day) supports muscle growth and recovery while limiting fat gain. Larger surpluses don't build muscle faster — past a point the extra just becomes fat, because muscle has a ceiling on how fast it can grow.
Calories permit growth; protein and progressive resistance training direct it. Without the training stimulus, a surplus is just weight gain. The macro split above starts protein high for this reason. If you're new to lifting or returning after a break, you can gain muscle even near maintenance.
As you gain weight, your maintenance rises — so a fixed calorie number that was a surplus becomes maintenance, and the gain stalls. Use the planner to model a goal weight with calories that recalculate as you go, or the tracker to calibrate to what's actually happening.
For most people 250–500 kcal/day above maintenance is enough for lean gains. Bigger surpluses add fat faster without adding muscle faster.
Beginners, those returning to training, and people with higher body fat often can gain muscle at or near maintenance. A surplus mainly helps once you're past the early stage.
Usually too large a surplus, too little protein, or no progressive resistance training. Tighten the surplus, raise protein, and make sure training is actually getting harder over time.