From Traderpedia

Definition: The invisible vertical line at the right edge of a price chart where anything to the left of the line is in the past, and anything to the right of the line is in the future

Everyone is interested in what happens to the right of the "hard right edge", unfortunately we've only got what happened on the left of it to base our trade decisions on.

The hard right edge is what eliminates hindsight from any trade decision. It is easy to eyeball a chart and think "I'd have been long right *there*" before that big bull run. Its a lot different trying to do the same thing as you watch the bars form on the right edge of the chart.