Executive functions are the mental processes that manage other thinking: planning, starting tasks, holding information in mind, switching between tasks and regulating emotions and attention.
Working memory, starting tasks, planning, sense of time and emotional regulation are the ones adults with ADHD describe most. Autistic adults often describe difficulty switching between tasks or adjusting when plans change.
Executive function is not the same as intelligence. Plenty of people who are highly capable at their subject can't start the email about it. That gap between ability and output is something adults tell us about often at assessment.
Executive functions depend on context. They tend to work better with interest, urgency or external structure, and worse with boredom, fatigue or overload. That's why the same person can be focused at 11pm before a deadline and unable to start at 10am.