An lvalue is a reference to a memory location where a value may be stored as a result of an assignment operation. An lvalue may be a script variable, an array, a string, or any other modifiable location in memory. Unmodifiable values such as constants, cannot be lvalues.
Here's an example:
lvalue = $x + $y;
Although the term variable also refers to a memory location where a variable can be stored, in a script it is a specific location for a specific value or purpose. lvalue is a more general term referring to the concept, rather than an exact location in memory.