A system call is a method for an application program to request an OS to perform some operation. Most modern OSes implement system calls by “software interrupts”.
The system call interface is a well-defined but “narrow” method to request an OS to perform some operation. It is also OS dependent. This means an application program designed to work with an OS may not work for another OS, unless the system call interface is abstracted as a library feature to hide differences amongst OSes.