Demonstrative adjectives point to a specific noun, corresponding to English "this/that" and "these/those": ce before a masculine singular noun (ce livre, this book), cette before any feminine singular noun (cette maison, this house), and ces before any plural noun (ces livres, ces maisons).
Before a masculine singular noun starting with a vowel sound, ce becomes cet for smoother pronunciation: cet ami, cet homme. French doesn't normally distinguish "this" from "that" — when the distinction actually matters, -ci and -là are added after the noun: ce livre-ci (this book here) versus ce livre-là (that book there).