I'm reading an wikipedia article:
(...) was sentenced to five years' probation and one year in the House of Correction (...)
five years' has an apostrophe? What does that sentence mean?
The English genitive (-'s for singular nouns, -' used on plural nouns; more commonly known as the possessive) can be used in some expressions of time to associate something with the length of time.
Hundred Years' War
one week's notice
a good night's sleep
a hard day's work
In your case, the genitive is denoting that the probation was five years in duration, with a similar construction to "Hundred Years' War" and "Seven Years' War" (those are actual names of wars). "One week's notice" is notice given during the week before something happens.