Colour gels are transparent coloured material used in photography to create special effects or moods, the coloured gel is placed in front of the flash so that the light travels from the flash, through the gel and towards your subject, illuminating the area with the desired colour effect.
Coloured gels are available in two basic types, colour corrected and non-colour corrected.
Colour corrected gels are tinted in order to compensate for daylight or tungsten so that the camera will see the light as white, this is particularly important were you have multiple sources of light at different colour temperatures.
Non colour corrected are obviously not colour corrected so you will need to understand and apply the colour wheel to get the correct output you are after.
A good quality colour gel should be heat resistant so that it wont melt or degrade under the heat of your lighting, therefore don’t be tempted to go cheap, buy a quality colour gel that is up to British standards.