Contents of the texts
The texts on the rongorongo tablets are in their nature scientific (in a broad sense). Similar to Minkowskis four-dimensional diagrams over space-time these texts map time-space.
We who live in cities, who are civilized, have lost contact with time-space. But think about events instead. Christmas is characterized by the event Santa Claus, at another time of the year there is an event with a turkey involved, at least in some places on Earth. In other places where customs are different also the events are different - at least on the surface.
In ancient times, however, they were not different in that way, because nature was everywhere making itself felt. No artificial lighting, no central heating, no McDonald's at the street corner. All events were not there at all times, light not in the 'season' of the night, warmth not in the season of winter, food not before harvest etc.
Therefore time, as expressed by fundamental events, was the primary, independent, variable. Seasons were like the dials on a clock, showing how time passed. To be able to predict events (just as in the hard sciences of today), repeated experiences were mapped, systematized, and made into a beautiful complete whole, a unified theory of everything. A cosmos.
Time was most easily observed in the Moon; its phases and where in the sky it was positioned told people much. During the day Sun gave similar directions. You could point to somewhere in the sky and promise to be home when Sun would arrive there.
The stars were needed when navigation started a very long time ago. The texts on the rongorongo tablets are though not necessarily originating with the Polynesians, they were not alone in navigating. Such experiences accumulated much earlier. Stars gave more exact directions than Sun and Moon.
Results documented in the rongorongo texts include mathematics (e.g. the relation between the circumference and radius in a circle), astronomy (e.g. 99 lunations for 5 Venus cycles), mythology (e.g. the turtle flipper injuring one of Hotu'a Matua's six men).
The texts should be read primarily as scientific 'formulas', expressing the results of firm evidence about how human reality works, the essence of the results of thousands of years of repeated natural experiments.
System of writing
Rongorongo is a symbolic picture system of writing. The pictures which were used as glyphs seldom mean what they show. Mostly what is meant is not what is shown, but just mnemonic aids for reaching (and teaching) the intended meanings. Plural because there are more than one frame of reference used in writing and reading these texts. The texts are not superficial.
Consequently rongorongo is a symbolic system of writing. The glyphs are similar to our system of special signs for representing numbers and other mathematical ideas.
But contrary to our alphanumerical systems, and indeed also contrary to the modern writing systems of the Chinese / Japanese and so on, the rongorongo glyphs should be understood both (at the same time) as pictures and as symbols.
Few among us recognize why the letter O is placed in the middle of our Latin alphabet - viz. because of its similarity with the Full Moon. The full moon is halfway between one black new moon and the next such. And who among us realize that the similarity between O and 0 is not a coincidence, understand why a similar symbol was chosen for representing zero? We have lost contact with the pictures which are at the roots of the symbols. We see just their dusty old skeletons.
The glyphs of rongorongo are symbols, but these symbols are pictures explaining their meanings (plural). Nuances of meaning are always seen in the way the glyphs are drawn. Glyphs - with very few exceptions - are not stereotypes, therefore they are not symbols in the strict sense. The glyphs are compositions where the parts are more of symbols than the whole glyphs.
Each glyph is a complex of signs telling a story.
The signs written are extremely exact and every detail carries meaning according to a simple and natural system of logic. Normally a glyph carries many such signs and it is therefore difficult to find two glyphs with exactly the same design and meaning. However, there is no need for nor indication of the glyphs carrying sound values. Sounds are generated by the inspired reader.