Software Development Kit is typically a set of software development tools that allows the creation of applications for a certain software package, software framework, hardware platform, computer system, video game console, operating system, or similar development platform.
Salesforce recently launched Mobile SDK 3.1 which includes some upgrades and enhancements by which you can enhance your native and hybrid mobile applications. It's an open source pack of technologies like REST API and OAuth 2.0. As mentioned earlier it supports native and hybrid app development.
Here is the link where you will find most helpful resources to get started.
In a nutshell
-> CocoaPods for iOS : You can now use CocoaPods to merge Mobile SDK modules into your apps. No more tedious downloading, copying, and re-configuring. CocoaPods does it all for you.
-> Gradle for Native Android Apps : Android Studio and the Gradle build system are now fully supported for native apps. Gradle support for hybrid apps will arrive after the next Cordova update.
-> iOS 7 is now the minimum OS version of iOS supported.
-> Raised the minimum Android OS version required by Mobile SDK to v4.2.2 (API 17).
->Supporting libraries upgraded to the latest versions
-> Flows to initiate authentication with an enterprise certificate
-> Unified SmartSync APIs (full offline support for application)