It's one of the simple ones: fห๐ฉ is f. And ๐จfห๐ฉ? It's f. Like the identity functions, Constant doesn't compute anything but just returns one of its inputs. It's somewhat different in that it's a deferred modifier, so you have to first apply Constant to its operand and then to some arguments for that non-event to happen.
The design of BQN makes Constant unnecessary in most cases, because when a non-operation (number, character, array, namespace) is applied it already returns itself: ฯห is the same function as ฯ. If you've used much tacit programming, you've probably written a few trains like 2ร+ (twice the sum), which is nicer than the equivalent 2หร+. However, a train has to end with a function, so you can't just put a number at the end. Applying ห is a convenient way to change the number from a subject to a function role.
+รท2 # A number 0.5 +รท2ห # A function +รท2ห 3 (+รท2ห) 7 5
When programming with first-class functions, the constant application shortcut becomes a hazard! Consider the program {๐จโพ(2โธโ) ๐ฉ} to insert ๐จ into an array ๐ฉ as an element. It works fine with a number, but with a function it's broken:
โ {๐จโพ(2โธโ) ๐ฉ} 1โฟ2โฟ3โฟ4 โจ 1 2 โ 4 โฉ M โ - m {๐จโพ(2โธโ) ๐ฉ} 1โฟ2โฟ3โฟ4 โจ 1 2 ยฏ3 4 โฉ
Here m is applied to 2โ๐ฉ even though we want to discard that value. Spelled as m, our context-free grammar knows it's a function argument, but this doesn't affect later usage. Under always applies ๐ฝ as a function. The proper definition of the insertion function should use a ห, like this:
m {๐จหโพ(2โธโ) ๐ฉ} 1โฟ2โฟ3โฟ4 โจ 1 2 - 4 โฉ