That is your adblue top-up warning.
It usually means there is a need to top up the adblue before it runs out. If it gets too low the car will (usually) prevent you from starting.
Adblue is a chemical that is mixed with the exhaust gas and reduces car emissions.
The mile count usually indicates how long you have left until the car will prevent you from starting.
As per the sign, you need to fill up with at least 2.5 gallons (I assume gallons) for it to properly register a top-up and no more than 4.
Some cars have a procedure to follow if you top-up due to a warning message. For example after filling up, power the car up but don't start the engine for a few minutes so the car has time to register the new volume in the tank.
But, all manufactures are different, so consult your manual.
Top-up options include using the large cartons of Ad-blue ( usually cheapest if you shop around) you see sold at garages and aldi. However, adblue systems can be a bit fussy and general advice is to avoid these if you can see they have been sat outside for weeks on end. Some say this can degrade the quality of the adblue.
The better option may be just to use a forecourt filler, many stations have them now (bp have good ones usually). You may pay a bit more per litre than the cartons, but it's easier, less messy and you can control the amount you put it.
If your display is giving you max/min top up quantities in gallons, don't forget to convert to litres. So 2.5 -4 gallons would be 11.3 - 18.1 litres.
Also ensure you are topping up the Adblue into the correct location, it is usually has a blue cap and is near the petrol/diesel cap. Do not fill it via your petrol/diesel filler.