Are they very safe and secure ?
Extremely. The Required Lockers (Hulk, Mummy, MIB, Rockit, Dragons, Forbidden Journey) use a fingerprint scan, and are free while you ride.
Go to a computer station, touch the screen, and follow the onscreen instructions. After you scan your finger, the computer assigns you a locker. Put your stuff in, then close the locker and press the green button to lock. They do lock automatically if you forget to press the green button. Most people do the "tug check" to make sure their locker is locked.
When you return, go to the same computer station, type in your locker number, then scan your same finger. Follow the on screen instructions and it will unlock for you. Locker are one-time use, so if you have to go back to the lockers for any reason, don't open your rented locker. Get a new one, since they're free and all!
At the Optional Lockers (Ripsaw Falls, Jurassic Park, Popeye), it's a fee--$4 for 90 mins. These lockers will ask you to pay first, then make up a six digit code (most people use their birthday). If you go over the 90 min time, the locker will stay closed. It will simply ask for more $. If you accidentally leave your $ in the locker and go over the time, simply ask the TM near the entrance to the ride and they will call for locker assistance.