Interest rate is a price of money. Interest rate is set by Central bank as part of Monetary policy. Interest rate is a interest rate at which money can be borrowed from central bank. Higher rate leads to less money borrowed which leads to less money being added to the economy. While lower rates lead to more loans and more money borrowed.
Relationship between interest rates and Inflation
Higher interest rates lead to lower inflation rates since less money is injected into economy.
Relationship between interest rates and stock market
TBD