A field placeholder prints a column of your data: on every label, the value of that label's own row. Put one in from the Fields menu of the text editor or of a barcode, or drag it from the Data source panel. The menu lists the label set's fields, each with an icon for the kind of values it holds; with no data connected yet, it offers Connect data source instead.

The Field settings
Field	Which column. Pick another to switch the placeholder to it.
Offset	For numbers: a value to add - negative to take away - as an amount (Absolute) or as a Percent of the value, so a price can print 10% higher. For dates and times: a number of Years, Months, Days, Hours, Minutes and Seconds to add or take away, for a best-before date 30 days after the production date.
Format	How the value is written. The list depends on the field: for numbers, Numeric formats, Currency formats and Percentage formats; for dates and times, Date and time formats; for yes/no fields, Yes/No, True/False or On/Off. Type your own, or see Number and date formats. Text fields have none.
Number and date format	Which country's way of writing it; Default format follows the Settings. See Placeholders.
Letter casing	As typed, or changed to lower case, upper case, or capitals at the start of the sentence or of each word.

What prints
An empty cell	Nothing, and the text around it stays. Turn on Compact rows in the Text section and a line left empty is left out.
A duration	A time span - from a database, say - prints as one: 01:30:00 for an hour and a half. The offsets lengthen or shorten it.
A picture	A column of pictures, as some databases store them, can't go in a text; use it in an Image instead.
Hexadecimal or Binary	For a number with decimals, the nearest whole number is written in hexadecimal or binary.
Emoji	Left out of a text, as they are when you type them; a barcode keeps them.
In a barcode	The value is encoded exactly as it is. A value the symbology can't take - letters in an EAN, say - shows an error on the work area instead of that label's barcode.

A field belongs to the data of its own label set. When you update the data and a column is gone, or remove the data source, the fields that used it are taken out; the text around them stays.