Source: Dungeon Master's Guide

Rod, uncommon (+1), rare (+2), or very rare (+3) (requires attunement by a warlock)

While holding this rod, you gain a bonus to spell attack rolls and to the saving throw DCs of your warlock spells. This bonus is determined by the rod's rarity.

In addition, you can regain one warlock spell slot as an action while holding the rod. You can't use this property again until you finish a long rest.