The Player's Handbook describes the different damage types on p. 196; the corresponding portion of the basic rules is here.
The PHB describes Force damage in such a way that it seems to be a solid magical manifestation that strikes your opponent in such a way to deal damage. But what separates this from bludgeoning damage?
How would I describe force damage to my players in a way as to justify that it bypasses a creature's bludgeoning damage resistance, but a normal bludgeoning attack doesn't?
Blunt force attacks—hammers, Falling, constriction, and the like—deal bludgeoning damage.
Force is pure magical energy focused into a damaging form. Most effects that deal force damage are Spells, including Magic Missile and Spiritual Weapon.
From the description of Force, even if you conjure a Spiritual Weapon that would normally do bludgeoning damage, say a club, it would still count as Force because it is of magical energy, which would bypass any bludgeoning resistances by nature.
Force. Force is pure magical energy focused into a damaging form. Most effects that deal force damage are spells, including magic missile and spiritual weapon
How force behaves is let vague and not particularly related to bludgeoning as it can manifest in many other ways.
For example if magical energy tears you into pieces by pulling your limbs, it can be force. Or maybe the connection between the atoms that constitute your body ceases to exist, or whatever you like as a GM.
Historically, insubstantial creatures in D&D were described as only partially being in our world (and partially ethereal), and thus had resistance to most damage types except force.
Border Ethereal DMG page 48: A traveler on the Ethereal Plane is invisible and utterly silent to someone on the overlapped plane, and solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal The exceptions are certain magical effects (including anything made of magical force) and living beings.
Note that Ghosts, Wraiths, Specters, etc. have: "Damage Resistances acid, cold, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical attacks". This means that even most (non-force) spells will generally only do half damage, for the above reason.
