What is this device?

[SirDarknight]

A fibre optic cable came from the ISP and got into this device and an ethernet came out and got into my router

 

[COLGeek]

Looks like your fiber "media converter":

http://ftthcpe.en.alibaba.com/product/60700062921-801305328/ftth_single_gigabit_port_catv_gepon_onu.html?spm=a2700.icbuShop.84.7.6d00858R05X38

 

[techgeek]

It's often referred to as an ONT (Optical Network Terminal).