Hi
SI is one of the Index Registers. SI is a 16 bit register which is native to the 8086, 80186, and 80286, and 80386-SX.
There are two Index Registers: the Source Index (SI) and the (Destination Index) (DI). they are used for string instructions, such as movsb/w, lodsb/w, etc.
The 80386-DX 80486, Pentium, and up, all the way to the last 32 bit Intel processor introduced an expanded set of registers that were, of course, now 32 bit.
The registers that were expanded included SI and DI. (ESI & EDI for 32 bit).
When used in conjunction with a Segment register (any of DS, ES, FS and GS) the index registers are able to point to data symbols/addresses in a different segment. Segmentation is out of the scope of this quick rundown, so you may check the wiki for more info.
Referencing SI, or DI, or their 32 bit counterparts, ESI and EDI, without an appended segement register implies that you are taking data from the current value of the Segment registers.
As a side comment: I don't think it's too wise to admit that you had to google to find the answers to the entry quiz: that defeats the point.
Welcome and whatnot, and enjoy your stay.