Sourdough is a living being and as such must be fed. It needs flour and water so that the bacteria contained within it can turn the starch of the flour into sugar and start leavening. Depending on how often you use sourdough and depending on where you keep it (in the fridge or at room temperature), it can be refreshed every day and up to about every 5 days.
If you use yeast very often, every day to make bread, you can keep it at room temperature, while if you use it 1-2 times a week to bake it, it is better to put it in the fridge, in the less cold part. Place it in a glass jar with the lid (better than the plastic ones) and calculate that after the refreshment it will have to fill it for no more than a third (even less).
During rest, the initial volume will increase by 2-3 times. For the refreshment it would always be better to use the same flour (the same brand) so that the yeast is as stable as possible: the more stable the yeast, the easier it will be to calculate the processing times for a dough. If you keep the yeast in the fridge, take it out about two hours before having to cool it down so that it is at room temperature. If a harder part or crust has formed on the surface, remove it and keep only the underside.
For the refresh follow the photographic steps of the post Sourdough refresh step by step.