Use the word straits when you're talking about a truly difficult situation. When you don't have enough money to pay your rent, you're in pretty bad financial straits.
These figurative straits are inspired by geographical straits, or water passages, that are too narrow to be sailed through. In life, you'll encounter straits that feel even trickier to navigate. If you don't study for your biology test, you'll be in dire straits, and if you forget to bring any food on your camping trip, you might find yourself in desperate straits. The Old French source of strait is estrait, "narrow part."