VASI stands for Visual Approach Slope Indicator. These are systems of lights used near runways (or anywhere else aircraft might land, such as aircraft carrier decks and helipads) which indicate to the pilot of an aircraft descending to land there whether they are on the proper glideslope, or else above or below it. The simplest VASI consists of two sets of two lights each, each pair perpendicular to the runway and off to one side of it. These lights (bright enough to see during daylight) are configured so that if the pilot descending at the proper angle, the two further lights will be red and the two nearer lights will be white (Red over White, everything's all right). If both sets of lights are white, the aircraft is above the glideslope (too high) and if both sets are red, the aircraft is below the glideslope (too low, and red means danger).
There are more complex versions of this, but they all tend to work roughly the same way. The precision approach path indicator (PAPI) is a line of four lights perpendicular to the runway. If the left two are white and the right two are red, the aircraft is on the glideslope. If the aircraft is slightly low, then there will be one white light at the left end, and then three red lights. If all four lights are red, the aircraft is significantly below the glideslope. Conversely, three whites and a red indicates slightly too high, and four whites indicates significantly above the glideslope.