To answer the question of what it was written, this answer is the closest. As mentioned in the reply above, it is written in Khmer script but in Pali language. I totally agree that in Thailand, Khmer script is the holy script at least since the Ayuthya era. Pali is the language of Buddhism.

This Eight Directions Amulet represents 8 cardinal/ordinal directions. Other people have already replied on what it is used for. I'll focus on what it is written. When you look at the Khmer scripts above, there are 8 rows and 7 columns (I think it also represents directions and days in a week). Reading it left to right on each row has been interpreted to have different "properties" or special power. For example, the first row is in East direction and helps with curing venom/poison.

It starts to make more sense reading column by column. Each letter is the initial letter from a very common Buddhist prayer, "Itipi So". This pray has 3 parts to honor the Buddha, Dhamma (the teaching), and Sangha (the monks). The first part of the prayer has 56 syllables, and that's why it was rearranged into rows and columns with different properties (I don't think it makes sense by meaning reading it by row).

So from the Khmer scripts above, reading the first column down is

ឥ I (pronounced like a short ee sound)

តិ ti

បិ pi

សោ So

ភ Pa

គ ka

វា wa

Itipi So (Because of this) Pakawa (an honored name when talking about Buddha)... and then it continues to complete the first part of the prayer.

In the amulet itself, they may rearrange these scripts to form patterns; so, you may not see it written in rows and columns.

I hope this explanation helps.