The Training Recovery Checklist
When you exercise, you break DOWN muscle tissue. How well you recover determines how well you rebuild that muscle tissue. It's a balance between doing enough to cause the breakdown and also adequate rest/recovery to rebuild back stronger.
Working out hard and not sleeping enough, failing to warm up/cool down, restricting calories (whether this is intentional or not), lack of stress management, and you'll plateau, struggle to see progress, or worse, develop overuse injuries.