Lautering is the physical separation of sweet wort from the solid spent-grain bed — usually accomplished by draining liquid through a false bottom, a slotted manifold, or a mesh bag, using the settled grain itself as a natural filter bed once the initial cloudy runnings clear.
The grain bed is doing double duty as a filter
Once a mash tun starts draining, the grain particles that settle at the bottom form a filter bed that clarifies the wort passing through it — which is why the very first liquid drawn off (before recirculating, a step called vorlauf in German brewing terminology) usually runs cloudy and gets returned to the top of the mash rather than collected, letting the settling grain bed do its filtering job before the real runoff begins.
A slow or stuck lauter is usually a crush or grain-bed problem
If wort stops draining or slows to a crawl mid-lauter, the most common cause is a grain bed that's compacted too tightly — either from a crush that's too fine, from a high proportion of husk-less grain like oats or wheat without enough rice hulls mixed in, or from channeling that's collapsed the bed unevenly. Adjusting crush coarseness or adding rice hulls for a husk-poor grain bill are the standard fixes, well before assuming equipment is at fault.