Custom Permissions let you define access checks that can be assigned to users via permission sets or profiles, similar to how you assign user permissions and other access settings
Example like :-
use it to provide access to processes,
use to involve in or bypass validation rules, etc
To check in apex :-
Boolean hasCustompermission = FeatureManagement.checkPermission('your_custom_permission_api_name');
Custom Permission User Case :- we need to bypass validation rule using Custom Permission for some user
Step 1 :- Go to the Setup Home, Quick find box Search for Custom Permission
Step 2 :- Click on Custom Permission, Click on New Button
Step 3 :- Fill the below details in the field values
Lable :- Bypass Account Validation
Name :- Bypass_Account_Validation
Description :- give accordingly
Connected App :- leave blank
click on Save
Step 4 :- again go to the Setup Quick find box search for Permission set, click and Open it
Step 5 :- click on new button, and fill the below details
Label :- Custom Permission set
API Name :- Custom_Permission_Set
Description :- give accordingly
click on save
Step 6 :- Now go to the Custom Permission APP section "Custom Permission", click on that and open it.
Step 7 :- Click on Edit button
Step 8 :- select the Custom Permission we created and click on save
Step 9 :- now click on Manage Assignments
Step 10 :- click on Add Assignment
Step 11 :- select the User which you want to assign, click on Next
Step 12 :- click on Assign
Step 13 :- click on Done
Step 14 :- now go to the Object Manager - Select any Object in our Case, we select Account Object go to the Validation Rule of the Account Object
Step 15 :- Click on New button in Validation Page in Account Object
Step 16 :- Give the rule name and update this validation "AND(ISBLANK(Phone),ISPICKVAL( Industry , 'Media'))" we checking Phone field,
when Industry has Media value if phone is null value give error
Step 17:- go to the Account create new record with Industry value as Media and Phone value as null and see the validation fire
Step 18:- Now we can use Custom Permission to over rule the validation rule for some profile, now go to the Validation rule and use this
"AND(NOT($Permission.Bypass_Account_Validation), ISBLANK(Phone),ISPICKVAL( Industry , 'Media'))"
Step 19 :- now go and check creating new record with industry as Media and Phone as null, you able to create new Record, this is the way you can bypass validation rule