Everybody laughed for some reason when I said that people eat KFC for Christmas dinner in Japan.
I feel like he probably doesn't even know what he's laughing at anymore, but he just can't help it.
It’s not like you to cry at movies. You always laugh really hard when there are any serious scenes in them.
You know that 笑う is "to laugh." This is the noun version, because the う in 笑う turns to い (so don't confuse this as an adjective). What is the noun version of "to laugh"? It's a laugh.
The reading is the same as 笑う, so just make sure you know that so you can know this as well.