That looks like some kind of fern (maybe something similar to Boston fern, although there are probably hundreds or thousands of different ferns out there) . However it's also possible the fern are in their own pots that are sunk in the soil around the perimeter of that large container so as not to compete too much with the tree roots (and make it easier to replace with new ones or different plants).

Normally underplanting citrus is discouraged because the citrus are surface rooters and you want to minimize competition for water and nutrients. Ferns usually grow in shadier locations so the ones in your pic are being shaded by the canopy of the tree the way they are planted.

Jenny, I saw the same type of set up with an oroblanco grapefruit. There was a label that said asparagus fern on this one and from the picture looks like it was grown right it the same dirt. All of their citrus had underplanting and all looked amazing which is why I thought about doing it myself.

Joe - it looks like there is a layer of mulch (or some type of barky mix) on the surface which could be masking any sunken pots. As a note - those trees are huge, in huge pots, and are most likely carefully fertilized.... and unless you have a large greenhouse, you wouldn't have a pot that large to try to underplant and be able to maintain nutrients for the tree. If you sink small pots around the perimeter, their roots would not compete as much with the tree's roots and could be fed essentially "separately".

The 2nd pic definitely looks like asparagus fern (so the original pic is probably boston fern -both pretty common)!