The example above says that a hurricane warning "is in effect". That means that there is currently an active hurricane warning.
Other things that can be "in effect" are laws, policies, rules, etc.:
There's a new "no-bag" policy in effect at Spartan Stadium. It's supposed to prevent people bringing weapons into the stadium.
Several states have laws in effect banning cell phone usage while driving.
When a rule, law, or warning first begins to be in effect, you say that it was "put into effect":
A ban on indoor smoking was put into effect in New York in 2003.