The Tagalog word si is what grammarians call a "personal topic marker." In Tagalog, you use it in front of a proper name. It's something you don't need in English, but you must remember to use it in Tagalog! 

Si Andrew
= Andrew

Si Andrew ay matangkad.
Andrew is tall.

Mahal ko si Teresa.
I love Teresa.

Si Mark ay payat.
Mark is thin.

Tawagan mo si Lisa.
Call Lisa.

The plural form of si is sina.

Sina Pedro at Eric
= Pedro and Eric

Remember:  in Tagalog you must always put something in front of a proper name! Compare this to the Tagalog words ni and nina, which are also used in front of names.