Phonics based reading is all about sounding out unknown words. The idea is that the student would understand if somebody else read the text out loud, so if we can teach the kids how to convert the written words into sounds, they can understand many new words they first come across. The core idea is to teach the kids that certain letters or groups of letters map to certain sounds (phonemes) at a start, and then gradually introduce more and more rules of English phonetics, allowing students to successfully learn to sound out even more complicated words.

The hope is that students will gradually learn to just recognize words by sight, which the overwhelming majority do eventually learn to do, and just need to sound out unfamiliar words. The fact that some students have struggled to learn to recognize words and need to sound most out is part of why people try to create alternatives, but those largely don't work well.

Of course, English does have some tricky phonetics. We have some words with multiple different pronunciations. We have some words with the same phonemes but different meanings that differ solely based on syllable stress. There are even some words whose pronunciation simply must be memorized, as there is no coherent rule to get from the word to the pronunciation (see for example Colonel).