The DR number, Diederich Rank, refers to the highest number in Diederich's frequency listing, which you can see here: Diederich Ranking.
DR 121. Malum quidem nullum sine aliquo bono. ~ Note: The words are adapted from Pliny the Elder's Natural History, 27.
DR 122. Qui capit, capitur. ~ Note: This is one of those "karma" proverbs, which expresses the idea that what you do unto others will be done unto you.
DR 122. Qui potest capere, capiat. ~ Note: This is a motto of the Gleg family. Note the subjunctive: capiat, "let him grab."
DR 123. Ite, si itis. ~ Note: You can find these words in Plautus's Poenulus.
DR 123. Ite et videte. ~ Note: You can find these words in the Gospel of Mark, 6.
DR 123. Terra es, et in terram ibis. ~ Note: You can see this phrase represented as a "word rebus" here: image.
DR 124. Quod tuum, tene! ~ Note: As often, the antecedent of the relative pronoun is not expressed: (Hoc), quod tuum, tene!
DR 124. Sua tenenda cuique. ~ Note: Here you have a gerundive expressing the idea of necessity; sua is neuter plural, hence the neuter plural tenenda. As usual, the dative - cuique - is being used to express agency. In English, you might say: "Each person should hold on to what is his" (or hers!).
DR 125. Deus ante omnia amandus. ~ Note: Note how the gerundive expresses the idea of necessity or obligation: Deus amandus (est), God should be loved...
DR 125. Amandi sunt nec multi nec nulli. ~ Note: Note the nec...nec... construction, which is equivalent to the English "neither... nor..."