Had is the past tense and past participle of have. It shows that someone possessed something or did something before now.
I had a dog as a kid means you owned a dog back then, not today. We had a wonderful meal in that restaurant means that we ate the meal in the past, and it is finished.
One nearby point: when had sits before another verb's past participle, as in had eaten, it forms the past perfect. That use is a different topic.
We had a wonderful meal in that restaurant. Here had means that we ate that meal in the past. The dinner happened, and it is finished.
He had a bad cold last week. At that time, the cold was with him. Now the week is over, and the cold may be gone.
Where People Mix It Up
Readers may expect had to carry extra meaning on its own, but sometimes that expectation does not match what the word does. It is simply the past form of have, so had a cold means a cold in the past, just as have a cold means one now.
For regular verbs, the past tense and the past participle have the same form, and this also holds for some irregular verbs, including have, whose past tense and past participle are both had. This shared shape does not change what the word means.