According to official Unity documentation:
What does this mean? Look at Figure 1.0.4a to understand its nature. The Tangent is a normalized vector that follows the U coordinate orientation of the UV on each geometry face. Its main function is to generate a space called Tangent-Space
This property is reviewed in detail later, in Chapter II, section 6.0.1, as well as the Binormals for the implementation of a Normal Map over an object.