To start it properly – and for you to understand what it means – a little history lesson:
On 29th October 2013 Google and Motorola announced thet they start work on modular smartphone and Danish designer Dave Hakkens (who announsed work on similar project in April of same year) will help them – that’s how ARA was born.
In first concepts every element of phone was nothing more then module – even SoC. But fast this become gigantic problem and Google/Motorola changed plas making motherboard (with SoC) un removable base. But still you could just pin in additional battery, increase internal storage, put camera or good speaker etc.
For some time it was silent about ARA, one of last informations we have are this advertising spot but we know that work was on quite advanced level – for this year tests was planned also developer kits shoul be avaible before end of this year and its launch was planned for 2017!
So what happened? We don’t know but it may have something to do with Googles person from “unifying hardware projects” – Motorola ex-president Rick Osterloh. Same persone that is blamed for changing Nexus series on Pixels.
Maybe ARA is just removed as individual project and it will be merged with Pixel – it looks like few pixels took together.

Source: Android Police