Pretty much as the description says, remove 14 cards in a row from the board, small tip though, take up any joker/wildcard cards you get offered via watching an advert, it would seem playing one during a streak does not count against you in the process, I managed to get a 15 card streak while playing a joker in the middle of it.
Edit: Some further good advice in the comments, be sure to have a read through
Edit2: Some people are reporting no longer being able to get free wildcards (platform specific?) Edit3: By the sounds of it you can only get a wild card on premium now playing endless
Edit4: OK, just sounds like wildcards drop less often outside of premium now.