In sitelen Lasina, letters have about the same sounds as in the IPA. Vowels are ideally a bit more centralized in Toki Pona than their IPA letters suggest. See Phonology.
Transcriptions in the IPA use 2 main kinds of brackets:
Square brackets mark pronunciation. Phonetics can be transcribed broadly or narrowly.
Slashes mark phonemic notation. A phoneme is a feature that distinguishes one word in a language from another. Symbols in phonemic notation can diverge from their phonetic values. For example, the English r phoneme can be written as /r/, even though phonetically, [r] would represent a trill.