When video is stored digitally in a YUV colorspace, Luma and Chroma information can be stored either interleaved to keep individual pixel components together or as planes, keeping individual Frame components together.
Packed Formats store chroma (U and V) information for Pixels either directly preceding or following the luma (Y) information for the same pixel. Because the spatial positions of all chroma samples line up directly with a single vertical position (horizontal line), chroma samples stored in Packed Formats can't be co-located (located between lines of pixels) as they can in planar formats.