What is Phonics? 

Phonics is the relationship between letters in written language and the phonemes, or individual sounds, in spoken language. Understanding this relationship will help children to recognize known words automatically and to decode unknown words. 

Systematic and explicit phonics instruction is better than non-systematic or no phonics instruction, according to scientifically based research. Systematic and explicit phonics instruction improves children's word recognition, spelling, and reading comprehension. It is shown to be beneficial to children from various backgrounds and is particularly beneficial for children who struggle to read. It is most effective when introduced early. Strategies used by systematic and explicit phonics programs include practice with letter-sound relationships in a predetermined sequence. Once students learn these relationships, they learn to decode words that include them. Other strategies include using these letter-sound relationships to write or read words, sentences, or stories that contain the relationships that they are learning. 

In the video below, Shantell Berrett of Reading Horizons explains more about systematic and explicit phonics instruction.