What does 'including but not limited to' mean?

It means that something is likely to include whatever is described, however, there could be more that is just not listed here.

For example, if I say that toxins that could be in product X include but are not limited to alfatoxin, botulinum toxin, etc., that means that alfatoxin and botilinum toxin are likely to be in product X, but there could be more.