This is the ‘model’ for speaking about time and tense put forward by Hans Reichenbach in Elements of Symbolic Logic, 1947.
It involves three moments in time: speech time, reference time and event time. These are what your text labels ‘S’, ‘R’ and ‘E’; I am going to call them ‘ST’, ‘RT’ and ‘ET’ for consistency with other answers I have written on ELL.
Think of your verb as a camera taking pictures of events on a timeline running from the past through the present into the future.
- The position where your camera stands is the point in time where you speak or write your sentence: ST.
(Note that in a conversation ST is ordinarily ‘now’, but if you are reading an old text that’s not the same as the ‘now’ when you are reading it. It’s the writer’s ‘now’.)
- The direction in which you are pointing your camera is the time you are talking about: RT.
- The position of the event you are photographing is the time at which the event occurs: ET.
In the English tense system, the ‘simple’ and ‘progressive’ constructions are said to be deictic (‘pointing’) tenses. They speak of events which occur at reference time, the time you are pointing your camera at. Your first sentence is an example.
I tried snails three years ago. . . . You are pointing your camera into the past (RT < ST) and speaking of an event, trying snails, which occurred at that time (ET = RT).
‘Perfect’ constructions, however, are relative tenses. They speak of events which occur before the time you are pointing your camera at.
I have tried snails. . . . Your reference time here, the time you are talking about, is the present: (RT = ST). But the event you name, trying snails, lies in the past: (ET < RT).
There is more about this at our tag-wiki on tense.
The advantage of this simple model is that it allows us to speak about time references in complicated utterances without confusing ourselves with terms like ‘past’, ‘present’ and ‘future’ which we use to label both times and verbforms.
You should be aware that there are other models which are derived from Reichenbach’s and use similar terms, but in different ways. CGEL, for instance, uses Tr, “time referred to”, as approximately the same thing as ET, but its To, “time of orientation”, and Td, “deictic time”, are not distinguished the same way as RT and ST; and it introduces an additional term, Tsit, “time of situation”, to incorporate discussion of grammatical aspect.