Great questions! Slices are a set of TPU chips that share a fast, private inter-chip-interconnect. Unlike the current GPU generation in clouds, the TPUs on different machines can communicate through this private network. Multislice means that we're using a hierarchical network, where there is both inter-chip-interconnect and normal data-center netowrking.
More details: https://cloud.google.com/tpu/docs/multislice-introduction
(P.S. - contributor on blog post, Google employee, all thoughts my own)