what does sample time specifically do?

hi, i'm trying to simulate a code in simulink using interpreted matlab function, but the code need to be executed at a fixed interval of time(let's say at 1khz).(i'm trying to write a pid code using this user defined function to implement in the embedded system) i guess the interpreted matlab function has some restrictions regarding the variables. is it done by setting the sample time? i read the document on sample time and i couldn't find out what it does. could some one explain me the concept?? i'm new at using matlab and having a little trouble doing it. please help out! thanx.