Tensor is to describe the invariant using an array or arrays.

The Euclidean Metric Tensor matrix in the middle:

When we double the basis vector(covarient), we can deduce the g matrix(metric tensor matrix) double twice, hence we call it double contravarient.

Metric Tensor matrix is critical elements in Einstein’s general relativity equation. But there are problems with Galilean Relativity. For example in electromagnetism, applying this Galilean transformation will conclude that Newton’s law doesn’t hold anymore. Until introducing Lorentz Transform, which contains length contraction and time dilation, caused by postulating speed of light invariant, the problem is solved.