raising and lowering of spin and angular momenta should be quite intuitive I guess, that it is an operation which increases or decreases the projection of a certain angular momentum about a fixed axis (the standard in almost all quantum mechanics is that one has the Z-axis as reference).
The length of the angular momentum vector is always fixed, and due to quantum mechanics, only certain projections on axes are possible (c.f with a "classical" vector which can have a continuous setup of projections)
A rasing, or lowering, operation just changes that projection, but of course by a disctrete amount.
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