Precision Approach Path Indicator. The name gives it away. They give guidance down to the ground if there is no ILS Glide Path. Harmonisation reduces as one nears the ground but at this point we're looking to flare aren't we.
When you next pull into your road at night turn off your headlights as you approach your driveway in case of dazzle.
See? Daft idea isn't it. UK invented PAPIs - UK keeps them on all the time.
SGC