A process is an execution of a code but a thread is a single execution sequence within the process. A process may contain multiple threads. A thread is sometimes named a lightweight process.

A JVM runs in a single threads and application in a JVM share the heap belonging to that process. That is why various threads may access the same object. Threads share the heap and have their own stack memory space. This is how one thread's call of a method and its local variables are remain thread safe from other threads. But the heap is not thread-safe and has to be synchronized for thread safety.