The JIT compiler is part of the JVM that speeds up execution by identifying 'hot spots' (code executed frequently) and compiling that bytecode into native machine code at runtime. This compilation happens only once, so subsequent executions of that code run at near-native speed. This is why Java applications often 'warm up' and get faster over time.
If a translator is reading a book to you and sees the same complex sentence repeated on every page, they eventually memorize the translation and say it instantly without consulting the dictionary.
Why does a Java application sometimes run significantly faster after it has been running for a few minutes (the 'Warm-up period')?