In English, as in many other spoken languages, you're used to distinguishing between singular and plural. As a computer language designed by a human linguist, Perl is similar. As a general rule, when Perl has just one of something, that's a scalar.[*] A scalar is the simplest kind of data that Perl manipulates. Most scalars are a number (like 255 or 3.25e20) or a string of characters (like hello or the Gettysburg Address). Though you may think of numbers and strings as different things, Perl uses them nearly interchangeably.
A scalar value can be acted on with operators (such as addition or concatenation), generally yielding a scalar result. A scalar value can be stored into a scalar variable. Scalars can be read from files and devices, and can be written out as well.