Sight words (high frequency words) are words that are known by sight - automatically.
These words are memorized by how they look (shape and letter order).
The more sight word vocabulary a reader has, the less work a reader has to do while reading.
A child with an expanding sight vocabulary is less exhausted by the reading process because he/she spends less time decoding words and more time concentrating on meaning and comprehension of stories.
A child with an expanding sight vocabulary is a more fluent reader.
Children with a low sight vocabulary require repeated practice - lots of it! Please see Mme Menard Blogspot - Let's Play A Game to find ideas for practicing sight words at home.