Final answer:
A variable is signed by default, meaning it can hold both positive and negative values. To declare a variable that only holds non-negative values, the 'unsigned' keyword must be used during its declaration.
Step-by-step explanation:
In the context of computer programming, when you define a variable without specifying whether it is signed or unsigned, by default it is considered to be signed. This means that the variable can hold both positive and negative values, just as numbers can have signs, either positive or negative, to indicate their relation to zero. A positive sign or no sign at all indicates a value greater than zero, while a negative sign indicates a value less than zero. To create a variable that can only hold non-negative values (zero and positive numbers), you must explicitly use the 'unsigned' keyword at the variable's declaration. For example, in the C programming language, declaring an int variable as 'unsigned int myVariable;' means that 'myVariable' can only contain zero and positive integers.