How field values are stored in the database
Different field types can have different data columns in the database. Most modules have a 'value' column. Others, like reference fields, have a 'target_id' column instead. The Date module can create a 'value1' and 'value2' column (for start date / end date).

You can't assume that it is always 'value'.

The 'target_id' value refers to the ID of the entity being referenced. So, if the field is referencing nodes, target_id is holding a node id value. If the field was referencing users, the value stored is the user id.