Learn Arabic (Modern Standard): لا ترفض العدل.

In Arabic, when you are talking about an abstract concept or general idea in its entirety, you must use the definite article ال. Because justice is a broad concept, it requires the definite article and becomes العدل.

Is عدل a verbal noun (masdar)?

Yes, عدل (justice or fairness) is a verbal noun from the basic Form I verb عدل (he acted justly). It uses a very common pattern for these nouns: three root letters with a short a sound on the first letter and a sukoon (no vowel) on the second letter.

How do we know لا ترفض means do not reject as a command, rather than you are not rejecting?

To tell someone not to do something, Arabic uses لا followed by the present tense verb. If it meant you are not rejecting as a simple fact, it would look exactly the same without vowel marks! Context usually makes it clear whether it is a negative command or a simple statement.

When speaking, how do we pronounce the transition between ترفض and العدل?

Normally, a negative command like ترفض ends with a sukoon (no vowel sound). But because the next word العدل starts with the definite article, Arabic adds a temporary short i sound to the end of the verb to avoid having two vowelless letters in a row. You would pronounce it la tarfuDi l-adl.