It depends on the problem, but here's an example.
Let's say you earn $20 an hour and you already have $100 in the bank. If you work x hours, then you'll have 20x additional dollars on top of the $100 you already have. So in total you have y = 20x+100 dollars
x = number of hours worked
y = total amount of money
This does not take into account taxes or other kinds of fees/deductions from your paycheck.
Comparing y = 20x+100 with y = mx+b, the slope is m = 20 and the y intercept is b = 100.
The slope tells us how fast the money increases (since it's the rate of change). In this case, slope = rise/run = 20/1 meaning that for each hour, the money goes up by $20.
The y intercept is the starting value which in this case is $100. If you worked x = 0 hours, then you'll still have that $100.