What does "If in doubt, sit them out" mean in practice?

It is the core rule of the UK Concussion Guidelines for Non-Elite (Grassroots) Sport, which apply to all grassroots sports from primary school age upwards, across England, Scotland, Wales and Northern Ireland.

• At all levels in all sports, anyone suspected of having a concussion must be removed from play immediately. You don't need to be sure it is a concussion.
• Once removed, they must not return to activity that day, even if their symptoms seem to clear.
• No-one should return to competition, training or PE lessons within 24 hours of a suspected concussion.
• They should be assessed by a healthcare professional, or through NHS 111, within 24 hours.
• In the first 24 hours they should not be left alone, drink alcohol, drive, ride a bicycle or operate machinery.
• They can only return to sport after following the graduated return to activity and sport, and never to competition before day 21.

Teammates, coaches, match officials, team managers, administrators and parents who suspect a concussion must all do their best to get the person off safely and quickly. Playing on can make symptoms worse, delay recovery and, if another head injury happens, cause more severe injury or, rarely, death.