If you're using Z Shell and a special prompt theme designed with Powerline fonts in mind, you'll need to install them on your machine. These are the most clear and cut-down instructions that I've found to work with Ubuntu 16.10 (also confirmed working for 17.04, 17.10, 18.04) and all credit goes to renshuki's Ubuntu 14.04 + Terminator + Oh My ZSH with Agnoster Theme gist. I've extracted just the Powerline font instructions - my personal setup uses Prezto instead of Oh My ZSH (not included here).
cd ~
wget https://github.com/powerline/powerline/raw/develop/font/PowerlineSymbols.otf
wget https://github.com/powerline/powerline/raw/develop/font/10-powerline-symbols.conf