In practice
Opening gaps mark prices where the market did not trade. Traders draw the gap between the close and the reopen and treat it like any other imbalance: an area price tends to return to, and a level that can act as support or resistance.
The weekly gap is given the most weight. Its edges and its midpoint are kept on the chart for the whole week as reference levels.
What to watch for
Most daily gaps are tiny and meaningless. A gap created by major weekend news can stay open for a long time, so trading purely for the fill is unreliable.