If someone has “A lot on their plate“, it means that they have of stuff they need to do. They are very busy. Someone could say, “I’d love to go for a holiday somewhere with you but I’ll have to wait until the new year because I have too much on my plate right now at work”.
Your back hurts. You usually sit a little hunched over, with your shoulders forward and your back bent. You don't think that's healthy, so you think this to yourself. "I need to work on my posture".
You and your friend are on vacation together. You're in a hotel room, and the bed was uncomfortable. Your friend asks how you slept, so you answer. "I was tossing and turning all night"
Why would you put an empty milk carton back in the fridge?
You've made a cup of coffee. You take the milk out of the refrigerator, but there's nothing in the container. You complain to your husband, who used the last of it.