What does "watch out" mean?

Why was "all" placed there?

When speaking, "all" is commonly placed after "you" to address more than one person. Ellen is speaking to her audience, which is a large group of people. Thus, she has added "all" to indicate that "you" is plural. Ex: Thank you all for attending this event tonight. (Plural "you") Ex: Thank you for your present. (Singular "you")

Is to say "I'm in love with you" different than to say "I love you?"

Good question! "I love you" is an expression that can be used in all kinds of relationships - between lovers, friends, family, and so on. However, "I'm in love with you" is only said to a romantic interest, and expresses that one has passionate feelings of love for someone else. Ex: I think I'm in love with my friend. What do I do? Ex: Stacy and Peter are so in love.

What's "pass by" mean?

"Pass by" means to go past, in, or near something on your way to another place. It can also mean for something to happen without fully noticing or experiencing it. Ex: The moment to go and talk to her passed me by. Ex: Let's pass by the shops on the way to Jerry's house.

What's "I bet" mean? Is this an informal way of speech?

Yes, it's a bit of a casual expression. You can use the expression "I bet" when you feel sure about/strongly expect something. You can also use it to mean that you feel so surely about something, you are willing to make a wager with someone that you will be right/win the bet. Ex: I bet I'm going to wake up late again tomorrow. Ex: The weather has been terrible recently. I bet it's going to rain again all week.

What does "your best" mean?

To do "your best" means to put in your best effort or work into something to try in succeed in doing something. "Trying your best" means that you put in the maximum amount of effort into something to try and succeed, even though you may not succeed. Ex: He didn't do well on the exam but at least he tried his best. Ex: I did my best to try and communicate in Spanish when I went to Mexico.