If you have an unstable currency it is bad for trade because:
People you are trading with will insist on dealing in a stable currency (not yours) such as the dollar or Euro. This gives you the extra cost and inconvenience of currency transactions, and the risk that you will, for example, buy your parts and materials on a Monday when the pound is low (meaning they cost you more pounds) and sell them on a Friday when the pound is high (meaning you earn fewer pounds). You will have to build a buffer into your buying and selling prices, or risk erosion of your profits.
You can try to buy protection against your unstable currency with forward contracts. This means you are paying somebody to take on the risk. They will make a charge which they can profit from, so you have an additional cost.
If you are not a trader, but a retired plumber hoping to buy a villa in Spain, you will need 15% more pounds now than you needed a year ago.
If you are living in Russia or Britain, and your currency is in a long decline, then your imported food will keep going up in price, exposing you to inflation.