"Phonological processes are patterns of sound errors that typically developing children use to simplify speech as they are learning to talk. They do this because they don't have the ability to coordinate the lips, tongue, teeth, palate and jaw for clear speech." according to  mommyspeech therapy .com- She explains it as well as I could.

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Final Consonant Deletion is when the child leaves the ending sound off of the word. This pattern should disappear by age 3 or so. If a child continues to leave the ending sound off of words, then the speech pathologist works on helping the child put the endings on his or her words.