Segmenting involves breaking words apart into individual sounds. 

When children segment a word, they hear the word, and then break it into separate phonemes, or speech sounds. Example: hat - /h//a//t/  Use manipulatives (chips, blocks, pretzels, coins, etc.) when counting sounds. Have children tap out the individual sounds in words. Ask them, “What sounds do you hear in dog?" (Say each sound, d-o-g, not letter name.)    
You can use "Elkonin boxes" to help children break the words into sounds. Click here for an example of this. Elkonin boxes