This is the Eolys fluid, which is used to burn off the soot particles
in the exhaust filter.
If you don't bother to do anything then the filter will block, and you
will have to get a new one, shouldn't be much over £500, unless you
can get it exchanged at about £200.
The fluid is available only from Peugeot or Citroen dealers, in a 3
litre top up kit, so you can do it yourself. It is expensive,
something like £25 per litre.
The tank which holds the fluid is just in front of the NS rear wheel.

NB. filling the tank will not turn off the warning, that has to be
done with the appropriate software.