I don't understand this. What's "heart" mean here?

Can you tell me some alternative words that can replace the word "explicitly"?

To replace “explicitly,” you can use the words “clearly” or “precisely.” Ex: The rules of the game clearly forbid communication. = The game rules precisely forbid communication.

I think "back" is used in many different contexts, but I'm not sure how to use "back" in the sentence. Could you give me some examples?

Here "back" references a past time, as if time were going "backward". It gives more context to the sentence! Ex: I was a part of the drama club back in high school. Ex: Back when I had my own restaurant, people came from all over town to try the food. Ex: Back in 1980, I visited the United States.

What is the meaning of "convinced"?

'Convinced' is the past term of 'Convince', which means to persuade someone into doing something. Ex: I convinced the coach to postpone practice.

I don't understand this. What's "heart" mean here?

Here "hearts" has two meanings! The first meaning is that hearts refers to other people that could love her, but here it also means the suit of "hearts" in a deck of playing cards. There's a continuous reference to "cards." The two people in the song call themselves the "joker" and "the Queen," which are also cards in a deck of cards. Ex: You need an eight of hearts to win the game. Ex: We're missing a joker card in the deck.

What does "at times" mean?

"At times" has the same meaning as "sometimes" and "on occasion" and can be used interchangeably with these terms. For example, you can change "they felt very sad and lonely at times" to "they felt very sad and lonely sometimes" without changing the meaning of the sentence. Ex: At times, I prefer to relax alone at home instead of hanging out with my friends. Ex: Although it was difficult at times, I persevered through school and was able to graduate with honors.