What is a Solenoid??

i have a 1997 Pickup XE.
i brought it to the michanic today because the check engine light was on, and there was other issues.
the bill: $1200. ugh. but its all legit. hes a great mechanic.

anyway, i have one question: what is the solenoid related to emissions? what does it do? (its not the starter solenoid)
curiously, it was simply missing from my car.

lol i just want to know what it does, so i am an informed consumer.

thanks for any help =]

it is a switch or a actuator..