[jwmerrill]

Both C and Julia are compiled to machine code before they are executed. C is compiled Ahead Of Time, and Julia compiles new code as it runs, Just In Time, whenever a function is called with argument types it hasn't seen before.

Parts of Julia's library and compiler are implemented in C, but this actually isn't very relevant to the speed of the generated machine code that actually runs.

Statements about Julia being "on par" with C mean that if you write code in a straightforward way to solve some problem, e.g. "find the three largest even integers in a collection," then Julia is capable of generating machine code that executes with efficiency "on par" with the machine code that C generates.

The "straightforward" part in the last paragraph is actually important. You could in principle solve this problem in any language by writing your own machine code generator in that language, and then the distinction between efficiency of different languages breaks down. But usually you won't do that, and so usually the distinction does have some meaning.