(cliché)  To wish luck or success for someone/something by crossing two fingers (middle finger over index finger) of one hand.

The weather channel is keeping track of the hurricane to keep the population informed. The hurricane is getting closer to our region, but let's cross our fingers that it goes out to sea

Il canale meteo sta seguendo il percorso dell'uragano per informare la popolazione. L'uragano si sta avvicinando alla nostra regione, ma incrociamo le dita e speriamo che si disperda in mare

We are going to be having a family picnic tomorrow. Cross your fingers that it won't rain

Per domani abbiamo programmato un picnic con tutta la famiglia. Facciamo le corna che non piova

One popular explanation dates it back to the early days of Christianity, when the religion was banned and Christians - to avoid persecution - would use this gesture as a way of acknowledging each other secretly.
In modern times, it lost its original religious symbolism, but the gesture still remains extremely common