In gnuplot, logical expressions evaluate to 1 or 0 as they do in C. Your function above will return 1 if (and only if) the `t`

you pass in equals 0 -- Otherwise it returns 0.

The documentation on `help operators`

states:

The operators in `gnuplot`

are the same as the corresponding operators in the
C programming language, except that all operators accept integer, real, and
complex arguments, unless otherwise noted. The ** operator (exponentiation)
is supported, as in FORTRAN.

Parentheses may be used to change order of evaluation.

I'm not really sure how much additional information there is to say. There is more help available for the various operators as well -- `help operator unary`

or `help operator binary`

...

Also see `help user-defined`

for a little more info on how to define functions and `help expressions`

for help on how to write gnuplot expressions in general.