An execution environment profile describes the capabilities of a Java runtime environment (JRE). In order to build for specific JREs, e.g. for a "Java SE 5" JRE or a "Java SE 7" JRE, Tycho allows you to select the execution environment profile to be used in the build.
Background: In OSGi, you can express that you need a certain minimum version of a library by importing a package from the library with a lower bound on the version. For the JRE itself, this approach is not possible: bundles don't specify imports on java.* packages, so you can't specify through a package import that, for example, your bundle needs java.lang in a version that contains the Enum class. Instead, there is the possibility to specify a requirement on the execution environment, e.g. Bundle-RequiredExecutionEnvironment: J2SE-1.5.
Execution environment configuration
Effects of the exectution environment in the build