A blockchain can only process its own data, no external api call allowed. Unfortunately a lot of interesting applications require access to external data. Thats what oracle are for. They import external data into the blockchains, embedded in transactions. Yes it does undermine the security model of the blockchain, but they are some work arounds. For example, you can have several oracles, and you can decided to only accept data that was provided by a majority of oracles, thus rejecting outliers.