Basically it's the idea that your higher conscious/subconscious is picking the cards to your question and that no card picked is a mistake or wrong... even if you feel like something doesn't make sense it may later on when events unfold. Each card has a few meanings, but tarot is a mostly intuitive practice. Usually a card reader follows not only the meaning of the card but also a mix of what feelings they get from the card.
A few things you should know is that tarot reading really doesn't have to do with anything psychic, so don't think you're being swindled or tricked or that you can't give away too much information. It's just answering your question through cards you picked out, so if your reader is having trouble and asks you questions it's best to answer honestly as it will make more sense why a certain card or cards are appearing. Also you should have some questions in mind before you go, but try to stray from yes or no questions or questions that are way too broad. You might want to look up "questions to ask for a tarot reading" just to give you some examples. Also whether you believe there is some subconscious power behind the cards you pick or not, it's honestly just a really good way to view a situation in a different light and maybe open your eyes up to something you've been missing, ignoring, confused on, etc. Oh I should also mention tarot usually answers questions in two ways, the outcome should you stay on the same path and the outcome should you change your path and how to change it.
Also I want to add sometimes you just don't get a good reader. I'd still try again if that happens because a really good reading is very exciting.