When I need a taxi, I need to say “打的” instead of "打车"。
So what is the meaning of "的"？How to understand "的"?
When can I use it?
You can say "打车". Whether to use "打的" or "打车" might be only personal/regional preference.
Here 的 is pronounced as di1 , and it shorten from "的士" which means taxi.
Currently I can think of only a few words which uses this meaning of "的": 打的, 的士, 的哥(male taxi driver), etc。
 My dictionary says the pronunciation is di2. While in everyday life (in Beijing), what I hear and use is di1.