This line is usually used as a joke to indicate that an actor is high-maintenance, but motivation is a very important factor for actors and characters. Motivation is what compels a character to do (or not to do) something, and if it's not clear enough, then the audience might have trouble believing in that character's actions – and maybe even the character themselves.
It's one of the reasons I enjoyed the novel of “The Hunger Games” more than the movie – because in the book we get to see Katniss' reasoning. She's smart and capable, and by being allowed to see her thought process, we know why she takes the actions she does.
When I'm reading stories, if a character does something that's... well... out of character, and it's never explained why, then I find myself dropping out of the story. I end up pausing to try and reason why they did this strange thing. Like, did I miss something that would account for this? Or was the author trying to be subtle, and in being so subtle they actually never explained it (note: this is why editors and beta-readers are important – they can pick up on things you might not notice).
I remember reading a story where a person had been transported from their home to a strange new land and this person spent almost the entire story wanting to go back home. The problem was that I couldn't figure out why they wanted to go back. By all accounts, their old life had been pretty terrible and their new life seemed really fun and interesting. Every time the character thought 'I have to go back', I found myself going 'but why?'. I couldn't think of a single reason why they were so determined to leave and they never provided me with one.
It even happens with television shows I like. Every now and then I'll find myself wondering why a character did something when they've learned better. Like, I love Supernatural, but certain episodes seem to be written by people who've never watched any of the previous episodes. A character's growth that was established in season 6 is suddenly thrown out the window in season 7.
I've been thinking about this a lot lately because I'm trying to adapt a Shakespearean play into something modern and I need to be clear about my character's motivations. I can't just go “Well, that's what the character did in the play”. That's not motivation – that's someone playing God with characterization and moving people around like chess pieces (which could work, if you wanted to play it that way).
We all know why Katniss volunteers for the Hunger Games (to protect her sister), and it makes it easier for us to relate to her and her choices. If you want your character to do something, then they need a reason – they need motivation. Why do they face off against the Big Bad Evil Guy instead of hiding under the covers, hoping someone else will step up? Why did they stab their best friend in the back? Why did they keep this super-important piece of information secret?
Remember that a character's motivations can change over time (and often do), and that this journey can be a compelling one to follow. Characters often start off selfish before growing to be more altruistic, but they don't just wake up one day and think “I now care about other people” – not without a reason. Heck, Scrooge needed 4 ghosts to teach him why he should care about others.
Now it doesn't mean that you need to explain every little tiny thing a character does, but if you've set up a character to be a cat lover and they suddenly adopt a dog, I'm going to need you to explain why.