“Locked In” is when a person is unable to move or speak, but their brain is 100% there, inside a body that can only communicate by blinking.    Maree Callis (https://www.youtube.com/watch?v=A3uEMyVnThI) explains it this way:

“I suffered a brainstem stroke which resulted in my being “locked in”. The best way for me to describe this type of stroke is to get you to picture a bridge over a river. On one side of the river you have “thinking” and on the other side you have “doing”. The bridge is the connector between the two. In my best Hollywood war jargon let me tell you – this stroke took the bridge out! This is a physical condition not a psychological condition. I am drug free, which leaves me with no brain cell loss. I have been blinking conversations for 17 years – this takes massive mental ability… I am 100% psychologically available.”