What it means
Call the police. 叫 here means "to call for."
When to use it
The number is 110 in mainland China (not 911). For an ambulance it's 120, and for a fire, 119.
What Chinese people actually say
Say this instead: 报警 (bàojǐng)
Chinese people almost never say 叫警察. The everyday word is 报警, literally "report to the police." You'd shout 快报警！ or say 我要报警. Use 叫警察 only if you're literally asking someone to go fetch a police officer.
Examples
kuài快jiào叫jǐng警chá察！
Call the police, quickly!
wǒ我de的qián钱bāo包bèi被tōu偷le了，qǐng请jiào叫jǐng警chá察。
My wallet was stolen, please call the police.
xū需yào要jiào叫jǐng警chá察ma吗？
Should we call the police?
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.