A threshold is what you step across when you enter a room. A threshold takes you from one place into another, and when you're about to start something new, you're also on a threshold.
A threshold is a point of departure or transition. Graduation can mark a threshold — when you graduate from circus school, you're standing at the threshold of your new career as a trapeze artist. Another kind of threshold is a limit or boundary. If you have a high threshold for pain, you can tolerate a lot of it, and your dentist can drill away without worrying about your squirming and howling.