Execution Mode for Process Builder and Flow

Process Builder and Flow Execution Mode

Process Builder runs in the system mode so the object and field level permissions both will be ignored for the user who triggers the Process.
However, if a Process is launching a Flow (which runs in system mode), the whole automation will run in the system mode.

Process Builder like Workflows runs in the System Mode hence not restricted by Field Level Security or object level permissions
Flows run in User Mode, Flows when Launched using a Process builder will run in the System mode.

for more detials:

http://theblogreaders.com/difference-salesforce-workflow-rule-process-builder/

http://theblogreaders.com/difference-system-mode-user-mode-salesforce/