Flu is an infectious and common viral illness spread by coughs and sneezes.
It's not the same as the common cold. Flu is caused by a different group of viruses. Symptoms tend to be more severe and last longer.
You can catch flu, short for influenza, all year round, but it is especially common in winter, which is why it is also known as "seasonal flu".
For more information see Influenza (Flu) on the NHS website.