PEMDAS (order of operations rules) require that we perform operations in a certain order: anything inside parentheses first, followed by any exponentiation, followed by mult. and div., finally followed by addition and subtraction.
Thus, we must evaluate 5+8÷4-1 first, as it's inside parentheses. Focusing on the division first, we get 5+8÷4-1 = 5 + 2 - 1, or 6.
Then we have 5 - [6], which comes out to -1.