what is the correlation coefficient (say between two stock returns over time)?

what is the correlation coefficient (say between two stock returns over time)? How do standard deviations and covariance interact in this equation (show equation) what is the correlation between stocks/bond market returns, and inflation/ inflation expectations/ interest rates over time? what does a-1, 0, and +1 correlation mean

"Get 15% discount on your first 3 orders with us"
Use the following coupon
FIRST15

Order Now