Languages have various levels of orthographic depth, which means that a language’s orthography can vary in a spectrum of a very irregular and difficult orthography (deep orthography) to a quite regular and simple one (shallow orthography).
English, French, Danish, Swedish, Arabic, Urdu, Tibetan, Burmese, Thai, Khmer, Lao, Chinese, and Japanese have orthographies that are extremely irregular, difficult and where sounds cannot be predicted from the spelling. These writing systems are more difficult and slow to be learned by kids, who may take years. In the medium of the scale, there’s Spanish, Portuguese, German, Polish, Greek, Russian, Persian, Hindi, Korean, where there are some irregularities but overall the correspondence of one sound to one phoneme is not that bad. At the positive end of the scale there’s Italian, Serbo-Croat, Romanian, Finnish, Basque, Turkish, Indonesian, Quechua, Aymara, Guarani, Mayan languages, and most African languages (because there were no history of spelling, so a new one of scratch was made as very regular), they all have very easy and regular spelling systems, with usually a one-to-one correspondence between and letters and sounds. These are very easily learned by kids.
Orthographic depth has several implications for the study of psycholinguistics and the study of language processing and also the acquisition of reading and writing by kids.