The daytime hours
There are three daytime hours: Terce, Sext, and None (pronounced to rhyme with "Bone"). They are named after the third, sixth and ninth hours of the Roman day, which began at sunrise and ended at sunset. The idea is that from time to time we should take a break from our labours and refresh ourselves with prayer.
These hours are much shorter than the others, and so they are often called the "Little Hours". Unlike the other hours, the expectation is that they should be prayed privately rather than with a group - minimising disruption to the working day.
In Universalis we have labelled these hours "Mid-Morning Prayer", "Midday Prayer", and "Afternoon Prayer", to indicate their times more clearly to a modern reader. Historically, the times have shifted quite a lot. At one time in the past, None got so early that the English word "noon" was born from it.
Not everyone can manage three breaks in the work of the day, evenly spread out. The rubrics for these Hours make allowances for people who may only have time for one of the daytime hours. If you have too little time, it is far better to take one hour slowly rather than to try to rush through three.
The structure of a daytime hour
There are three short psalms (or pieces of psalms), each with an antiphon. There is a verse or two from Scripture with a short versicle-and-response to encourage reflection on it. The hour ends with a short prayer. [Note that you won't see all the versicles and responses on Universalis yet: the missing ones will be added soon].
The choice of psalms
Partly for simplicity, and partly because there would not otherwise be enough psalms to go round, the choice of psalms at these hours follows a unique pattern.
Each day has a set of psalms assigned to it, enough for one daytime hour. If you only recite one daytime hour in the day, these are obviously the psalms to use.
On the other hand, it would be rather boring to recite the same set of three psalms three times over, once for Terce, once for Sext, and once for None; so there is a second collection of psalms, called the "Complementary Psalms", which are different for each hour but don't change from day to day. These psalms were written as songs for the Jewish pilgrims to sing on their way to the Temple.
So if you recite only one Little Hour, things are simple: use the psalms of the day.
If you recite more than one Little Hour, use the psalms of the day for one of the hours and the complementary psalms for the others.
Universalis shows you both sets of psalms, so you can choose.
A couple of final complications: some solemnities have no "psalms of the day", so the complementary psalms are used throughout on those days. And on a few days there is one hour at which the complementary psalms can't be used because they've already been used in Lauds or Vespers. Of course Universalis adjusts its pages automatically when these things occur.