This property appears to offer a refinement on the “justify” value used in the ‘text-align’ property. Indeed, the “justify” value must be set for that property for ‘text-justify’ to have any effect.
‘Text-justify’ offers a fine level of justification control over the enclosed content, allowing for a variety of sophisticated justification models used in different language writing systems.

div { text-align: justify; text-justify: newspaper }

This is “Newspaper” justified content

Possible Values
Value Description
auto The browser will determine the appropriate justification algorithm to use
distribute Justification is handled similarly to the “newspaper” value, but this version is optimized for East Asian content (especially the Thai language.) In this justification method, the last line is not justified.
distribute-all-lines Behavior and intent for this value is the same as with the “distribute” value, but the last line is also justified.
inter-cluster Justifies content that does not have any inter-word spacing (such as with many East Asian languages.)
inter-ideograph Used for justifying blocks of ideographic content. Justification is achieved by increasing or decreasing spacing between ideographic characters and words as well.
inter-word Justification is achieved by increasing the spacing between words. It is the quickest method of justification and does not justify the last line of a content block.
newspaper Spacing between letters and words are increased or decreased as necessary. The IE reference says “it is the most sophisticated form of justification for Latin alphabets.”