What is bid-mass?

Its the order of what you do things in a maths calculation
B=brackets
I= indices (powers)
D=division
M=multiplication
A=addition
S=subtraction

Example

What is 4 + 2 × 3?
If you calculate the '4 + 2' part first you get:
4 + 2 × 3 = 6 × 3 = 18
If you calculate the '2 × 3' part first you get:
4 + 2 × 3 = 4 + 6 = 10
These are two very different answers, but only one is correct.
In BIDMAS, multiplication comes before addition, so multiply 2 by 3 first.
4 + 2 × 3 = 4 + 6 = 10, so this is the right answer.  

Example

What is 9 - 4 + 3?
This sum has only addition and subtraction. So work them out from left to right.
9 - 4 + 3 = 5 + 3 = 8
Notice that if you didn't go from left to right you would get a different answer:
9 - 4 + 3 = 9 - 7 = 2
But that would be the wrong answer. Always work them out from left to right.