Definitions

Draught, normal (T)

The depth of water needed to float the boat, when it's loaded to its design displacement and sitting on its design waterline.

Draught, minimum (T_min)

In a boat with retractable appendages (rudder, daggerboards, outboard or sterndrive engine), the depth needed to float the boat when all these appendages are fully retracted. Again, this measurement is based on the design waterline.

Units: Linear measure (usually metres/centimetres or feet/inches).

Which one to use?

Most of the time, the maximum (normal) draught, plus a comfortable margin, is the figure of interest. T_min is relevant only for a handful of boat types, and then only in those specific conditions when you can safely run with the keel, rudders or engine retracted. Very few retractable keel monohulls, for example, can be sailed with the keel up (although many centreboard boats can run downwind with the board retracted).