Signedness is a property of data types representing numbers. A numeric variable is signed if it can represent both positive and negative numbers, and unsigned if it can only represent non-negative numbers (zero or positive numbers).1
As signed numbers can represent negative numbers, they lose a range of positive numbers that can only be represented with unsigned numbers of the same size (in bits) because roughly half the possible values are non-positive values, whereas the respective unsigned type can dedicate all the possible values to the positive number range.1
[1] Signedness, Wikipedia. (2019). https://en.wikipedia.org/w/index.php?title=Signedness&oldid=931665752 (accessed March 10, 2020).