A register is a part of the processor that holds a bit pattern. Processors have many registers.
MIPS processors have 32 general purpose registers, each holding 32 bits. They also have registers other than general purpose ones.
The first example SPIM program puts bit patterns representing integers into two registers. Then it adds the two patterns together.
The newest version of Spim is called "QtSpim". If you have not installed it already, do so by following the instructions in the appendix.
The picture on the right shows how to start SPIM on Windows 10. Find the box "Type here to search" in the lower left of the workbench. Type "QtSpim". The first result should be the QtSpim program. (Of course, this assumes that you have installed it.) Then click on the icon.
How else can SPIM be started?