The weekly open is a flat line that everyone can compute. Price above it means anyone who bought at the start of the week is ahead; below it means they are not. That crude framing is the whole idea.
It is used most in forex and crypto, where continuous trading makes daily boundaries less meaningful, and it is often paired with the monthly open and session-vwap as a set of neutral reference prices rather than levels with any predictive claim.
Its value is context, not signal. A market that opens the week and immediately trades away without looking back is behaving differently from one that oscillates across the line all week, and noticing which is happening is more useful than any rule about crossing it.
Related: prior-day-high-low, session-vwap, overnight-high-low, anchored-vwap, higher-timeframe