Slot 1 refers to the physical and electrical specification for the connector used by some of Intel's microprocessors, including the Pentium Pro, Celeron, Pentium II and the Pentium III. Both single and dual processor configurations were implemented.
It depends upon the phone. The exact support for each SIM slot depends on the phone but in most cases: SIM #1 supports all features of the network (2G/3G/4G/data) SIM #2 can be used only to make or receive voice calls, in many cases 2G only.