The aff. is shorthand for affinis (affinity) and is used to indicate an unnamed plant that bears a close relation or a strong resemblance to a specific species, in this case Heterocentron floribundum, but it's not that species as the plant displays some features that don't match the species.
Another abbreviation is c.f. which means confer (compares with) meaning that the plant, again, bears a very close resemblance to but shows some small features that don't match the standard species or cultivar description.
These abbreviations basically mean that the name of the plant is unknown or it may not have a name yet.
Too bad too for I have some great pictures of this one. I am finding lots of the pictures I take at the San Francisco Arboretum and the Conservatory of Flowers are not any good for the PlantFiles because they are either weird hybrids or just not named. Sad because they are so neat to share.
Anyway Baa, this isn't even the one I had a question about the other day. I still need to look for that one.
Thanks again and thanks Liz for asking a great question for me.
In cases of unidentified plants like the one in your image, we would encourage you to post them on your journal if you want to make them available for others to see. The information is still relevant and interesting, who knows someone may well come forward with more info!