MRS means Marginal rate of substitution it refers to that rate at which consumer is willing to substitute a good for other one.
It tends to decline when we move along the Indifference Curve...
Because to law of diminishing marginal Utility...
Example:
At point A and B consumer is willing to give up 1.5 units of good Y for one more of good X...
But as move.
The consumer will give up 0.75 unit of Y for X.... so it declined