Reduce decreases the Size category of the target creature by 1. This affects the maximum size of creature it can Throw or use as an Improvised Melee Weapon, and vice-versa. For example, a creature of Medium size can only be thrown by a Large or larger creature. When Reduced, a Medium creature becomes Small and can be thrown by a Medium creature.
While not stated in the spell description, Reduce also decreases the weight of the target creature. The formula used is:
Original Weight / 2.35 = New Weight (minus additional weight, if needed, to place creature in the correct size category)
The change in size from the spell stacks with the change in size from Disguise Self, making it possible to become a tiny creature (small race + Reduce). As a tiny creature it is possible to go through even the smallest of passageways such as pipes or jumping through the bars of a fence, something even the small races can't do.
The incantations for Reduce are Minimus ("Smaller") and Diminue ("Diminish").
Disadvantage on Strength Checks and Saving Throws.
1d4 less damage (minimum of 1 damage). It has 
Saving throws.
Action + 
Level 2 Spell Slot
Range: 9 m / 30 ft
Concentration
Duration: 10 turns 
or use as an 
, and vice-versa. For example, a creature of Medium size can only be thrown by a Large or larger creature. When Reduced, a Medium creature becomes Small and can be thrown by a Medium creature.