The U-value indicates the thermal insulation, i.e. how well a product insulates (keeps heat and cold out). The lower the U-value, the better the product insulates.
What defines a good U-value depends entirely on where the door or window will be located. In old houses where walls and ceilings leak a lot of energy, a U-value of around 1.0 - 1.3 is sufficient to increase comfort indoors. If, on the other hand, you build a passive house that is to be self-sufficient, the U-value should fall below 0.80.
However, it is not enough to just look at the U-value. Also other values such as the G value and
daylight transmittance are factors that must be considered in this context.
The G-value shows how much energy is transferred through direct solar radiation. The higher the value, the more energy and thus heating it generates (windows account for approx. 30% of heating in houses).
To think about here is how much heating you want via windows to the house. If you have large glass areas in dense newly built houses, it may be good to look at a lower G-value in order not to have to cool the house down on hot sunny days. Also consider the position of the window in the direction of the wind, and how much solar energy reaches the window.
The daylight transmittance is stated as a percentage and is a measure of the amount of daylight that enters through the window. A high daylight transmittance is preferable, then it is perceived as brighter.