Turn signals are lights which are a manual safety feature intended to allow drivers to inform other drivers around them of their movement/direction change intentions. They typically flash on one side of the vehicle, which indicate the area the driver intends to go.
Turn Signal/Indicator Locations
In most geographic locations, the minimum required by law is a turn signal light in the front left, front right, rear left, and rear right. However, some vehicles have them in additional places, such as embedded in the mirrors, or on the side of the body. Some truck trailers have turn signals along the side at the bottom due to the langth of the trailer. Many modern cars have the turn signals embedded within the headlights and taillights, while some have them in their own section of the body.
LED Turn Signals
Some turn signals come from the factory using light emitting diodes, and others may have retrofitted LEDs.
Many modern cars are equipped with LED turn signals from the factory for both life span and style benefits.
Many companies produce LED replacements that plug in where normal bulbs would be installed.
The Ambiguity of North American Rear Turn Signals
While most of the world uses amber turn signals on both the front and back of the vehicle, North America allows and uses red turn signals on the rear.
Blinker Fluid: Not a Real Item
"Blinker fluid" is used as a joke item with people that don't know enough about cars to tell that it isn't real. Another common item like this is a "muffler bearing".