Hi,

You can find all the theoric information about the the tensor in the link attached. But to resume, a Tensor is a representation of a matrix for n dimension in one vector using some transoformations (More details in the link).

In the recent deep learning frameworks: Tensorflow and Torch. The use the concept of tensor in order to represent the input image. It permit to make the process faster if the data is represented by tensors.

There are some applications for face and facial expression recognition using these frameworks. You can find several papers about that.

Hope it will help you,

Regards.

https://en.wikipedia.org/wiki/Tensor