What is the meaning of this? This is regarding the Array.prototype.concat method, but it's stated throughout the spec, again and again, for a series of methods.
The concat function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the concat function can be applied successfully to a host object is implementation-dependent.
Can you explain this in simple wording, perhaps providing an example as well?
It means that you can apply the method to an array-like object even if it's not an Array. These include NodeLists and the arguments object.
I sometimes use this feature to turn the arguments object into an array: [].slice.call(arguments) returns the argument list as an Array object, giving me native access to all the array methods.
You can read about this on MDN.