Set of instructions in Java are known as bytecodes. These instructions are interpreted by Java Virtual Machine (JVM). Bytecodes concept makes Java portable because only JVM needs to be installed in any system to interpret bytecodes. Bytecodes also makes it secure because only these codes can be interpreted by JVM and no other codes will be interpreted. It also makes a program run faster.
Just In Time (JIT) compiler is a compiler built by Sun Microsystems in Java 2 release. It is actually a part of the Java Virtual Machine (JVM). JIT runs in real time and is used to compile parts of bytecodes to executable code. Only parts of bytecodes are compiled to executable codes and not the entire program. This process is used to increase the performance of the compilation.