Evapotranspiration
How much water left the turf — evaporated off the surface plus transpired through the plant — usually reported in millimetres or inches per day.
Reference ET is a modelled figure for a standard short grass surface under the day's weather. It is not a measurement of your soil and it is not a crop requirement; it is the demand side of the ledger, and what you applied is the supply side.
There are several ways to compute it. Penman–Monteith is the reference standard and needs solar radiation, humidity and wind. Hargreaves needs only temperature and the date, which is why it is the practical choice when the only weather you have is a forecast feed — it is less precise and it is honest about being an estimate.
Comparing applied water against ET over a fortnight is more useful than either number alone. A run of days where you applied 60% of ET is a defensible deficit-irrigation decision; the same run when you thought you were keeping up is a surprise in August.
Your soil probes beat any model. ET is for the weeks between probing, and for showing a city council what the water actually went on.