z(1) and z(2) mean the same as they would in any code - there is an expectation that z is an array with (at least) 2 values, otherwise it will error.
The z in @(t,z)
is what you must pass in to call that function, known as an anonymous function, which requires two input arguments, t and z, although since t appears to be un-used it is un-necessary. You would call it as e.g.
z1_dot( 7, [2 3] )
