"have had" is Perfect Tense Form of the verb "to have".
Forming the perfect tense of any verb is by adding that verb in the participate form to verb "to have" ( verb to have + the verb in participate)
Verb Perfect tense
eat have eaten
take have taken
laugh have laughed
The perfect tense means that the action started in the past but could be JUST terminating NOW when the sentence is said , or has just terminated a short while ago.
The difference between the past and the present perfect , that both their actions start in the past , but the past tense action is fully terminated in the past and the far past ,while the present perfect tense is associated with actions that started also in the past , but mostly the near past and has just ended or might be extending in the present.
In your example :
I have had my lunch . ( means i have just had my lunch few mintues ago , i have just finished it ) while using the past tense , " i had my lunch" ( would mean i had it an hour ago , yesterday ,anyways the action of having lunch is fully terminated and not in the near past)
I hope you will understand what i have just written now !:)