This cartoon by Matt from the Daily Telegraph relates to the latest cut in interest rates paid by UK banks. Falling interest rates have hit pensioners hard. Many pensioners rely on the interest they receive on their savings to top up their pension, but with the Bank of England's base rate now standing at 1.5% this extra income will be dramatically reduced.
1. If you save money for a rainy day, you are saving it for a time in the future, when you might need it.
2. "Shrank" is the past tense of the irregular verb "to shrink" (shrink, shrank, shrunk). If something shrinks, it becomes smaller. Clothes often shrink when they are washed. The joke is that the money has shrunk because of the rain.