Traditionally, it is a year and a day period of bonding physically and spiritually with your significant other. At the end of that period, the couple can decide to stay together with a further dedictation, or part ways without trial or drama.
Unfortunately today, folks tend to see it as a legal pagan marriage.
It's a testing period to see if you can handle being with them for a (hopefully) life long marriage.
I, personally, think it's a good idea to try out before you marry someone. Although, most people these days don't understand the difference between love and lust. Which, I guess, would help by doing this.
If you're not serious about the other person, don't try it. Being bound spiritually to another is not something to take lightly, no matter the length of time it last for.