If these are at the tread rubber sidewall junction they are not a problem at all. Time and exposure to things like ozone lead to the drying of the rubber and small cracks appear in this area where small amounts of the sidewall rubber and tread rubber combine in the vulcanizing process.
If the cracks are in the main part of the sidewall this can be a bigger issue and the tire shouldn’t be used. Air leaks can develop.
If cracks are in tread rubber themselves then tire has most likely been exposed to tire treatments and it has degraded the rubber and should not be used.