What does the tolerance value for a trimmer or pot mean?

Using Arduino General Electronics
1

Does this mean each trimmer will vary from one another, or

does it mean that, once set, the resistance could change that much?

2

No, it means that the marked value (1M 100k etc) could vary by that much.

3

There's, also, another parameter [other than the Tolerance] called the Temperature Coefficient, that indicates how much the Total Resistance will change due to temperature--usually in Parts Per Million [PPM].

4

AWOL:
No, it means that the marked value (1M 100k etc) could vary by that much.

from unit to unit, not over time. Its short for "manufacturing tolerance".