As others stated, it is when two words in different languages are similar phonetically/graphically.

They are called false friends because they seem familiar and you expect to mean the same as with other words that are pronounced similarly in two languages and mean the same. False friends not just mean different things but they sometimes can change the phrase in a bad way.

Note: The following examples are with Spanish from Spain, they might be different in other Spanish speaking countries.

Alice and Bob discussed about having children
Alicia y Bob discutieron sobre tener niños

In English Alice and Bob had a conversation, in Spanish they had an argument.

Bob is a sensible guy
Bob es un tipo sensible

Bob is a person who acts with prudence and thinks carefully in English, in Spanish he is a guy who is easily affected by feelings.

I had an English teacher who told the same anecdote about false friends and her sister. She was in UK and the batteries for the remote died, so she went to a convenience store and asked if they had them.

The thing is that in Spanish we only use battery (batería) for the ones that are not meant to be replaced regularly, like the car battery, the laptop battery, etc. The replaceable ones (even the rechargeable that take the same shape), are called pilas. So in the anecdote, the sister told the woman at the store very confidently “Do you have piles?” (haemorrhoids, for the ones who doesn’t know).

I don’t know how real the anecdote was, but it fulfilled its purpose to make us aware of these situations.