Hi 

think that you have $10 in your bank account and you spend $12. 

you had $10 in you bank account but you spent $12 so now you are in dept of -$2 dollars that means you owe the bank $2 to not be in dept. 

any thing to the to the left of 0 on a number line is a negative number and any thing to the right of 0 is positive. Negative numbers look like this -1 or -5 or -10 and positive numbers look like this 5 or 10 or 13 ect. 
hope this helps.