Post meta is a data set in WordPress that contains additional core as well as custom post information.
Post meta is a data set in WordPress that gets saved inside its own table named {$wpdb->prefix}_postmeta. It contains the following columns:
┌──────────┬──────────┬────────────┬──────────┐
│ post_id │ meta_id │ meta_key │meta_value│
├──────────┼──────────┼────────────┼──────────┤
│ 1233456 │ 1234456 │ foo │ bar │
├──────────┼──────────┼────────────┼──────────┤
│bigint(20)│bigint(20)│varchar(255)│ longtext │
└──────────┴──────────┴────────────┴──────────┘
The post_id is needed to do JOINs, so the {$wpdb->prefix}_posts table can connect to this table.
The data that gets saved here by core:
_edit_lock_edit_last_thumbnail_id
This table also saves the data entered in custom fields.
And it's as well meant to hold additional custom data. If you want to search by custom field with a meta_query, it's better to use a single entry per value. Else you save it as serialized array (which core will do on it's own).
