I know a lot of words, but I often can't remember them. Like, I was talking to my dad on the phone one time and I couldn't remember the word 'box'. I ended up describing it as a 'container that you insert things into and it has a lid and...' which he eventually got. But, if my brain is on form, I can talk about and understand relatively complex words. Oftentimes, however, it's just not on form, though!