Some infants start to understand what "no" means at around 6 months, but most won't stop what they're doing in response to the word until they're somewhere between 12 and 18 months.
When your baby gets into mischief, calmly tell him no and try to interest him in something else. For example, if he keeps trying to grab the cat, distract him with a toy, move him away from the cat, or put the cat in another room.
If you feel as though "no" is the word you say most often to your baby, try using words that explain why you don't want your baby to do what he's doing. So when he gets too close to the oven, say, "Hot!" The stern tone of your voice gets across the most important part of your message.
It's also important to have places in your home where it's safe for your baby to play. Cordon off unsafe areas with safety gates, plug electrical sockets, and make sure dangerous or breakable items are out of reach.
The BabyCenter Editorial Team. (2013, July). When will my baby understand “no”?. Baby Center [Online]. Available: http://www.babycenter.com/408_when-will-my-baby-understand-no_1368459.bc. (2013, July).