Terminology
Whoever measures something compares two quantities with each other. If, for example, I want to determine the length of a table, I hold a folding rule against the table and read off its scale how long the table is. The folding rule is the measuring instrument, the table is the measurement object and the length is the physical quantity. A physical quantity that I measure is always compared with its unit.
Length has the unit metre. The unit is defined uniformly worldwide; it is fixed. For this purpose, the length of a metre is derived from natural constants so that it is the same everywhere on Earth. We intuitively rely on units always being the same size everywhere.
Measurement technology is about comparing physical quantities on site with their units and obtaining as little deviation as possible between the measurement result I have achieved with my measurement and the true value of the quantity. If, for example, the table is actually lWAHR = 73.05 cm long, that is the true value (however I determine it). If my folding rule shows the length lMESS = 72.9 cm during the measurement, this value is the measured value. We call the deviation between the measured value and the true value the measurement deviation.