For protection,you nedd to know 2 things -:
- User Space
- Kernel Space.
What is User Space?
User Space is the address space where User program resides.
Let a Process $P_{i}$ is using user space $1000$(address of that location and $P_{j}$ is using $1004$.Now $P_{i}$ is not supposed to access memory data./information of process $P_{j} $and also not of kernel space .
For this purpose protection is provided,so that $P_{i}$ should use only its own address space not of other process neither the kernel.
How this is implemented ?
Using the concept of limit register and base register.
See this
If $P_{i}$ tries to use other's address space ,then it move to trap /Addresing error.