You'll often hear musos talking about a neck reset (or neck set) on acoustic guitars. As this is something that tends to be required on pretty much every steel string acoustic eventually, it's worth understanding why it happens and what's involved in a neck reset.
The top of an acoustic guitar is under a lot of stress. String tension exerts quite a pull on it. Over the years, the wood of the guitar top succumbs to this tension and begins to belly a little at the bridge. This effectively raises the bridge and, with it, the action or string height along the neck. As the guitar ages, it becomes harder to play.
The solution is usually a neck reset. The neck is removed and some wood is removed from the heel to modify the angle at which the neck joins the body. This basically alters the geometry of the neck/guitar to compensate for the higher bridge. It's a pretty big job but one that most acoustic guitars will need at some stage in their life (when depends on a lot of factors) if you own them long enough.
If you think your guitar's action has been (very slowly) increasing over a number of years, you might have a candidate.
I've written an article for Guitarless outlining the reasons and the neck reset process so feel free to take a look - it goes into a bit more detail.
Incidentally, if you're considering buying a used or vintage acoustic, it's worth asking if it's ever had a neck set (which is a good thing if it's been well executed) and/or checking if the guitar needs it. If it does, you should factor that into your bargaining process as it can be a relatively expensive job.