The reliability is an indicator how reliable the suggested amount to buy is, with 10 being the highest and 0 being the lowest. This is a weighted average of the following subscores:
Lead time: high reliability if the deliveries from this supplier are always on time. Low if the deliveries come at a different time than the set lead time or at varying times.
Safety stock: high if the demand is very predictable. Low if there is a lot of variance in the demand, for example in order size or time between orders.
Forecast: high if we have good data to forecast and the forecast isn't too different from the history. Low if the product is relatively novel or has been out of stock recently.
Taking this into account you can evaluate how much you can trust the supplier in question.
If you find the reliability low, it is always a good idea to check the settings for the supplier.
There might have been a change in the lead time that you are not aware for instance.