It's a way of visually dividing a track up into different musical parts, Intro, Chorus, Outro etc. It can help with making mixes sound better but it's not essential.
You don't have to reanalyse your collection, you only need to reanalyse if you want to use that feature.
Here's the guide...
https://rekordbox.com/_app/files/img/rekordbox5.1.0_Phrase_Edit_operation_guide_EN.pdf
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