A light verb is a verb for which the semantic charge or load is "light". In other words it can be used anywhere, c'est un verbe à tout faire. Here "have" replaces more precise (or semantically heavy) words like eating, inviting, carrying, growing, or experiencing.
- We're having tuna casserole tonight.
- They're having us over for dinner.
- I'm having a baby.
- I'm having contractions.
The above are all events, though because be + ing represents either current or future time in 1-3, only the 3rd and 4th examples refer to an event that is in motion as the speaker speaks.
- We're having trouble finishing.
- *I'm having