Phrasing is the part of mixing that deals with when you start and subsequently mix in the records. Most dance music has 16, 32, and 64 bar phrases - therefore, when mixing records, you want to start the second record at the beginning of the live record's phrase. For example, after the drop, that is generally the beginning of a phrase of that record, so that is a good point to begin the second record. The second record will most likely be started at the beggining, which is another phrase point. Also to be considered in phrasing are where vocals are (so they don't overlap on two records), if the two records are in tune (if they sound bad when played together), when the bassline comes in, etc. When mixes are phrased well, they sound continuous, and people dancing to it find it easier to flow because there are no akward ???? moments.
