Unkown meta_value in ACF

For every meta value saved there is a second row prefixed with _ that stores what field definition this value refers to. Do not delete it.

Your example additionally is a repeater which makes it more complex but the docs has a nice explanation:

$ParentName_$RowNumber_$ChildName

So you have a repeater field called carousel which has a subfield called image and this is the first row:

carousel_0_image

As said an additional row called _carousel_0_image is saved by ACF to know which field this relates to. It is (if I remember correctly) some index relating to a Custom Post Type that stores ACFs Field definitions.

As a closing note: ACF has support and third party questions usually get closed here as off-topic, I am just responding because I knew this off the top of my head.

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