A back tick (`) followed by a series of characters represents a symbol, which is not the same as a string. See |Q for Mortals for more information about this.
q)s:`symbol q)s ~ "symbol" 0b
A back tick without characters after it represents the empty symbol. This empty symbol can be used with $ (cast) to cast a string into a symbol, creating symbols whose names could not normally be written down, such as symbols containing spaces. `$x is shorthand for "S"$x
q)s:`hello world 'world q)s:`$"hello world" q)s `hello world
q)t1:t2:(a:1 2;b:3 4) q)update a:neg a from t1 a b ---- -1 3 -2 4 q)t1~t2 1b q)update a:neg a from `t1 `t1 q)t1~t2 0b