Help with ACS switch and microcontroller (arduino)

[vectorex]

(Sorry for my English)
Please, help a newbie.
Few months ago I started playing with electronics and I ran into a problem which I couldn't solve or at least understand.
I've got some ACS120 switches/triacs from ST and wanted to use them with Arduino to switch on/off something in my house (say water pump, light, etc.).
But I need help understanding how to connect them together properly to avoid burning out my arduino board, as I don't understand the schema on Fig.14 (basic application) from ACS120 datasheet (http://www.st.com/content/ccc/resource/technical/document/datasheet/ab/bb/71/82/14/79/4f/6f/CD00003319.pdf/files/CD00003319.pdf/jcr:content/translations/en.CD00003319.pdf)
Could someone explain me how it works?

PS: On the forums people said I should go with optocouplers, but I think the circuit from the datasheet is way simpler. Just need to understand how to manage it to work.