The Symphony of Lights is a famous multi-media light show, making use of 50 buildings and landmarks on both sides of Victoria Harbour and held nightly at 8pm. During the programme, viewers can see lasers being shot into the sky from various fixed locations along with building facades projecting LED displays. The show lasts around 10 minutes and is accompanied with an audio music program which you can download as a mobile app through iTunes and Google Play.
The program started in 2004 and has been wildly popular with tour groups over the years. In all honestly, it is decent and interesting if you are in a location where the light show can be viewed. Otherwise, it can be disappointing when weather is poor or crowds intense or if the right expectations are not set beforehand. Just know what you are in store to see beforehand and you'll probably enjoy the programme - at least somewhat.
Good places to watch the Symphony of Lights include the TST Promenade and Avenue of Stars as well as the West Kowloon Promenade to a lesser extent on the Kowloon side. On the Island side, the Central Waterfront, Tamar Park and Bauhinia Square as well as some of the newer promenade development spots to the east and west. There are good views at some of the higher floor restaurants and sky bars on both side of the harbour as well.
In short, my recommendation has long been to stop and watch the Symphony of Lights if you are around one of the locations mentioned above around 8pm at night. Otherwise, if you have other plans, just do that instead.