Most high frequency words can be segmented and then blended back together in just the same way as every other word. However, because the early high frequency words (shown here) are short words and encountered constantly, it is possible and helpful for them to be learnt as sight words.
Sight words are those which are learnt as a whole: read without segmenting and blending.
Problems occur when children are given long lists of high frequency words to learn by sight, out of all context.
There are only a very few words which need to be learnt as a whole word: