What does [:] mean?

It is an example of slice notation, and what it does depends on the type of population. If population is a list, this line will create a shallow copy of the list. For an object of type tuple or a str, it will do nothing (the line will do the same without [:]), and for a (say) NumPy array, it will create a new view to the same data.

It might also help to know that a list slice in general makes a copy of part of the list. E.g. population[2:4] will return a list containing population[2] and population[3] (slicing is right-exclusive). Leaving away the left and right index, as in population[:] they default to 0 and length(population) respectively, thereby selecting the entire list. Hence this is a common idiom to make a copy of a list.

well... this really depends on the context. Ultimately, it passes a slice object (slice(None,None,None)) to one of the following methods: __getitem__, __setitem__ or __delitem__. (Actually, if the object has a __getslice__, that will be used instead of __getitem__, but that is now deprecated and shouldn't be used).

Objects can do what they want with the slice.

In the context of:

x = obj[:]

This will call obj.__getitem__ with the slice object passed in. In fact, this is completely equivalent to:

x = obj[slice(None,None,None)]

(although the former is probably more efficient because it doesn't have to look up the slice constructor -- It's all done in bytecode).

For most objects, this is a way to create a shallow copy of a portion of the sequence.

Next:

x[:] = obj

Is a way to set the items (it calls __setitem__) based on obj.

and, I think you can probably guess what:

del x[:]

calls ;-).

You can also pass different slices:

x[1:4]

constructs slice(1,4,None)

x[::-1]

constructs slice(None,None,-1) and so forth. Further reading: Explain Python's slice notation