How does tensor product/multiplication work in TensorFlow?
You may want to read the documentation.

output[..., i, j] = sum_k (a[..., i, k] * b[..., k, j]), for all indices i, j.

For instance, in your example 

$~~88=1\times12+2\times14+3\times16,~~~94=1\times13+2\times15+3\times17$
$214=4\times12+5\times14+6\times16,~229=4\times13+5\times15+6\times17$