System Integration Testing usually involves multiple systems and the output of one system acts as an input for another system. For Eg: System B is dependent on system A for input X to process in to output Y.
If system A is working properly it would generate Input X which in turn will be consumed by system B to produce output Y. This type of testing would be know as SIT.
During this testing phase a tester may uncover issues which wouldn't be possible if he/she would go on to test the component individually. It is an important phase of testing where companies having individual teams just to verify the same.