One line, two answers
Look at 3 + 4 × 2. One person adds first: 3 + 4 = 7, and then 7 × 2 = 14. Another person multiplies first: 4 × 2 = 8, and then 3 + 8 = 11. The numbers and the signs are the same, but the two answers are different. The line needs a way to say which part is worked out first.
Brackets do that. Whatever is inside a pair of brackets is worked out first, before anything outside them. (3 + 4) × 2 says: add 3 and 4 first. The bracket becomes one number, 7, and the line becomes 7 × 2 = 14.
(3 + 4) × 2 is two lots of 3 + 4: 3 + 4 + 3 + 4 = 14 squares.
Move the brackets, change the meaning
Now put the brackets around the other two numbers. 3 + (4 × 2) says: multiply 4 by 2 first. That gives 8, and the line becomes 3 + 8 = 11.
The numbers and the signs have not changed. Only the brackets moved, and the answer went from 14 to 11. In (3 + 4) × 2 the × 2 repeats the whole of 3 + 4. In 3 + (4 × 2) it repeats only the 4, and the 3 is added once.
3 + (4 × 2) is 3, and then two lots of 4: 3 + 4 + 4 = 11 squares.
Any operation can go inside
Brackets work the same way whatever is inside them. In 2 × (9 − 5), work out 9 − 5 = 4 first, and then 2 × 4 = 8. In (12 + 6) ÷ 3, work out 12 + 6 = 18 first, and then 18 ÷ 3 = 6.
Each time, the bracket is worked out and turns into one number, and that number takes its place in the line. Only then does the operation outside the brackets happen.
Two ways to go wrong
The first is to ignore the brackets. In (3 + 4) × 2, multiplying 4 × 2 first gives 11, which is the answer to a different line, 3 + (4 × 2).
The second is to stop too soon. In (3 + 4) × 2, working out 3 + 4 = 7 finishes the bracket, but the × 2 outside it still has to be done: 7 × 2 = 14.