Tactile pressure sensor and drift. Reading the datasheet
Tentative answer: 

There may be some relaxation mechanism, the way a dent will appear in a carpet if you leave a chair leg sitting on it too long. This specification places an upper limit on the effect that will have on the reading.

So if you read 100N force after 1 second, the reading may have drifted by 5% or 5N (or less) after 10 seconds, 10N (or less) after 100 seconds, 15N after 1000 seconds and so on. (I am assuming they mean log(base 10) but the datasheet is not explicit on that point)

It's not obvious if there is a lower limit to this behaviour (i.e. 15% difference between 1ms and 1 second) but I would assume so, down to the 5 us specified settling time. 

Another question is : does the same drift apply when you then remove the weight? (the chair leg depression disappears eventually after you move the chair). If I were contemplating this sensor the first thing I would do is get my hands on one and characterise it with actual measurements. It may be better than the spec, bearing in mind that future production may vary within these limits.

Whether this is good or bad depends on whether it meets your needs, and how well it compares with other sensor technologies within your budget.