Every drawing needs a datum
Why measurement has to start somewhere, and what happens when it does not.
A datum is the one fixed point or surface on a drawing that every other measurement is stated from, because a measurement on its own only makes sense as a distance from somewhere, and without an agreed starting point, two people measuring the same part can each be technically correct and still arrive at completely different places.
A distance needs two ends
Every dimension on a drawing is really a statement about the distance between two things, and stating a distance without saying which two things it is measured between leaves it open to more than one honest reading. A datum solves this by naming one surface or point on the part as the fixed starting place that a whole group of dimensions is measured from, so that twenty millimetres always means twenty millimetres from that same named place, and never twenty millimetres from wherever the last measurement happened to leave off. Without an agreed datum, two features can each be dimensioned correctly relative to different, unstated starting points and still end up in the wrong position relative to each other, because nothing on the drawing ever said the two measurements needed to agree about where they began. On a formal drawing the datum surfaces are labelled with capital letters, usually A, B and C, and a dimension that refers to datum A is a promise that it was measured from that one face and from nowhere else, whoever picks up the tape.
Why a tape hooked on one end stays accurate
Marking several points along a long plank of wood by always hooking the tape measure onto the same end and reading straight off it keeps every point accurate relative to that one end, however many points are marked, because each measurement starts fresh from the same place. Marking the same points by instead measuring each new distance from the previous pencil mark, walking the tape along the plank one step at a time, lets any small error in reading or marking carry forward into every measurement after it. Each step can be measured reasonably well and the last mark can still end up well out of position relative to the end of the board. A drawing without a stated datum works like the second method, every dimension technically correct on its own and still capable of drifting relative to everything else on the part.
The arithmetic is short. A reading error of one millimetre, repeated across six measurements each taken from the last mark, can put the final mark six millimetres out in the worst case, which is enough to miss a bolt hole entirely, and it is built out of nothing but small, individually reasonable mistakes. Hooked on the fixed end, the same six readings are each still only a millimetre out at most.
Choosing the surface that counts
The datum is usually whichever surface the part will actually sit against or be held by once it is in use, so that the measurements on paper match the way the part will really be positioned. A part measured from a surface that has nothing to do with how it is mounted can be dimensioned perfectly correctly and still fit badly once installed, since the drawing was describing distances that never lined up with the ones that mattered.
The same choice carries through to inspection. A part is normally measured from the same datum the drawing specified, since measuring it from a different, equally reasonable starting point can make a correct part appear to fail, or a faulty one appear to pass, purely because the two sets of measurements began in different places. A drawing with two competing datums, each group of dimensions measured from a different surface, is often harder to work from than one with no stated datum at all, since it looks complete while hiding the same ambiguity a missing datum would have made obvious. A workshop faced with two competing datums has no way to tell which set of dimensions takes priority if the two ever disagree, and that disagreement, however small, tends to surface only once the part has been made and no longer fits where it was supposed to go.