A process is basically a program that's running in memory on a computer. On modern computers, it has its own address space and other resources that the operating system provides for it. A process can have multiple threads, which are paths of code that's executing at the same time. All threads in a process share the address space of the process and can see each other's data.
When you copy & paste that exact question into Google, you'll get a lot of answers with much more detail.