Can anyone me, i don’t understand this?

It means that when you have same signs example : 1 + 2 ( they both have a positive sign ) we add them and keep the positive sign so 1+2=3 , and another example : -1-2 = -3 ( since both 1 and 2 have negative signs so we add them , and we keep the negative sign in the answer )

As for different signs , we subtract and we keep the sign of the biggest number for example : 5 - 3 = 2 ( since the 5 has a positive sign while the 3 has a negative sign , we subtract and 5 is greater than 3 so the answer will be positive just like the 5 ) ... On the other hand , if we have :

4 - 10 = - 6 since 10 is greater than 4 and the 10's sign is negative so the answer's sign will be negative too .