The plus sign is used in this case as a unary operator. -5 uses the minus sign as a unary operator and it is necessary to distinguish -5 from +5. We don’t need the unary operator plus sign because we take it for granted that +5 is the same as 5. So, assuming that the asterisk means multiply then the expression is the same as 3×5, which of course is 15.
These two unary operators take on a different meaning when they are placed between two numbers. The prefix bi- means two, and when two numbers are separated in this way with an operator between them, the operator is called a binary operator. So 2+3 means add 2 and 3, and 2-3 means subtract 3 from 2, so the operator acts on both numbers. And we can write 2+(+3), where one plus is a unary operator and the other is a binary operator. And we can write 2-(+3). In the first case it’s the same as 2+3 and in the second case it’s 2-3, so the unary plus sign is unnecessary and can be removed.