How does weed get you high and what is actually happening to you when you are?

The active ingredient in weed is the chemical THC, a so called cannabinoid. The human body has its own cannabanoid called anandamide, which acts as a painkiller among other effects. THC mimics anandamide, and smoking weed floods you brain with cannabinoids, especially the hippocampus (interfering with short term memory) and basal ganglia (interfering with body movements).

THC is of course only one of many other chemicals, many others which also can affect one's high.

Real ELI5: Your parents give you one snickers bar a day to keep you happy, however you save up money and go to your local candy store and buy a whole bag of mars bars and eat all of them on the same day. Eating them all at once is really delicious!

Well I didn't really mention what cannabinoids do:

First off, you might have heard of dopamine, a chemical that is active in the brain's pleasure and reward system. Do something that is feels good? Here have some dopamine. Eating delicious food? Dopamine. Listening to an awesome song? Dopamine. Get butterflies when you see a really cute girl, and eventually, if you play your cards right, have the best sex of your life with said girl? Dopamine, dopamine, dop...

Now you cant walk around in complete bliss all day so the brain has chemicals called inhibitors that eventually stop the release of dopamine.

Cannabinoids inhibit the inhibitors, allowing more and more dopamine to be released.

More cannabinoids -> more dopamine -> relaxation, stuff seems funnier and less important, you get hungry etc.