When someone says 'packing a lot of things in', it's actually a turn of phrase. Pack, as a verb, means to fill.
You have a backpack and when you place things inside of the backpack, it's packing. When saying 'packing a lot of things in', it means you are putting a lot of 'something' into another thing.
So in this instance, they mean they are putting a lot of knowledge into that lesson. You could also think of putting all that knowledge into your brain. It's quite a lot so you have to 'pack it in'.
'Packed in' or 'packing in' can also mean 'forcefully putting in something'. When you have too many clothes to put in your suitcase, you 'packed it in'. As in, put it in forcefully.