Meaning the time the Cash markets (ie the stock exchanges) are closing in Europe/UK.
Is of interest because it can create unexpected/unusual market movements at what is the middle of the day in the USA.
Normally Europe/UK cash markets close at 11.30am USA ET, but this does vary a couple of times a year when daylight saving changes occur at different times across the Atlantic.
Europe/UK close is 5.30pm Europe time (CET
) which is 4.30pm in the UK.