How are the human teeth related to the tusks of elephants?

The human teeth and the tusks of elephants are homologous structures.

Homologous structures are those structures that have same anatomical structure but perform different functions.

Human teeth is specialised to cut, tear and chew the food. Elephant tusks are specialised incisor teeth that are used for both digging up the food and combat as required for survival in the wild. So probably, both share an ancestor who had incisor teeth. 

Homologous structures are considered a missing link among a set of species who once were closely linked but developed into two distinct species due to evolutionary processes. However, their origin remains the same.