Interpretation of b1: when x1 goes up by one unit, then predicted y goes up by b1 value.
Here we need to be careful about the units of x1.
Say, we are predicting rent from square feet, and b1 say happens to be 2.5. Then we would say that when square feet goes up by 1, then predicted rent goes up by $2.5. This would be interpretation of b1 in this case.
Now, say x1 is listed as square feet in hundreds. Then we would say that when x1 goes up one unit, i.e. when square feet go up by 100 square feet then predicted rent goes up by $2.5.