In the UK, pants are women's undergarments. You might also call pants "knickers" or "underwear."
In most English-speaking countries, the garments you wear under your clothing are known as undies, underpants, or panties, but in the UK, they're pants — especially when they're worn by girls or women. If you talk about pants in the US, people will think you mean jeans or trousers, but a question like, "Do you want to see my new pants?" might be a little shocking in Britain.