Simply, a symbolic reference generally uses the value of a variable¹ as another variable's name.

# A simple example from http://perldoc.perl.org
$name = "foo";
$$name = 1;            # Sets $foo

# $name is evaluated ("foo") as the name of the variable
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Which is almost always NOT what you want to do (unless you really understand symbolic refs and have a good reason to do so). So if your code doesn't run under use strict 'refs'; you are accidentally using symbolic refs. This is why, using strict is often harped on.

While you are learning perl, it is much better to know how to avoid symbolic refs than knowing the details of them.

This is most likely your best reference.