That's actually one of the more practical spells in the standard Hogwarts curriculum, I think. The way it's described, 'Colloportus' creates a magical seal on a door or window, making it impossible to open through mundane force. It's not just a physical lock; the mechanism seems to involve a kind of adhesive or binding charm that fuses the moving parts together. I've always imagined it producing a brief, squelching sound on casting, like a suction cup or a wet seal being set.
Its limitations are interesting though. It's a basic defensive charm, so a competent witch or wizard could likely undo it with 'Alohomora' or a more powerful blasting charm. It feels like a spell for buying time or securing a room against Muggles or lesser threats, not for creating an impregnable vault. The magic feels temporary and dependent on the caster's focus and power.
You see it used a few times, like when Harry and the others try to seal doors in the Department of Mysteries. It's functional, not flashy, which fits its nature. I picture it as a sticky, magical glue that loses potency over time or under sustained magical pressure.