If you mean, Promote a post, it will basically give your post prominence within the page of all your followers, and their followers if the post is liked.
Its certainly not as effective as using the standard advertising with FB ... but also is a lot more cost effective with relation to spend (one of fee less than $10 as opposed to an unlimited spend if you wish)
If you have lot of followers (likes) already,. it can be effective, if you are trying to build likes on any page, it may not be as effective.
I aint one to advocate people spending money on adverting with FB or Google if they don't need to, but if you are really trying to build followers with FB, and wish not to put in the hardwork by organically growing your page followers, FB's paid advertising certainly can help, as you can target your market specifically.
We used it for a client over one week period limiting his spend to $10 per day, targeting a very niche market, with the distinct objective of building likes in a trial scenario, and within that week, we grew his like from 84 to above 350 (an audience of less than 8,000 targets).
Hope that helps

