In the spec for the ledger format, there's something called "Close Resolution" in the header. It is described as "the resolution (in seconds) of the close time." What does it mean, and what is its significance?
Ledgers do include a close time because it is expected that contracts will be able to support things that can "expire" such as escrow. However, validators might see a ledger close at very slightly different times owing to their slightly different network views and errors in their clocks.
To resolve this, Ripple uses a close time resolution. Validators announce the close time they actually saw to help their clocks stay in synch, but they only try to reach a consensus on the rounded close time.
If validators ever fail to reach a consensus on the close time (and this has happened a few times), they agree to disagree, indicate this in the ledger, mark a close time one second greater than the previous ledger, and (if it keeps happening) raise the close time resolution to make it more likely they'll agree.
If validators consistently do agree on the close time and the close time resolution hasn't reached the minimum (10 seconds, I believe) it will decrease until it reaches that minimum.