An EPQ is a really long essay - I think 5000 words? - about whatever you want lol. Some people related it to their subject so they can talk about it in interviews and personal statements. For example someone who wanted to do computer science at uni did it on "Can the world exist without Google" or something to that effect, someone who wanted to do motorsports did it on "Car Safety in Formula One".
When we went to the UCAS convention last year everyone we asked had no idea what an EPQ was so I wouldn't bother if you don't think you'd have time but it gives you some practice with researching and citing sources and referencing things properly which are all things you'd have to do at uni.
I didn't do it because of that, but it's worth more than an AS and less than an A level in terms of UCAS points so could come in useful when you need a boost and could get you alternative offers for some unis.
Basically if you have the time and motivation go for it but if you're just in the mindset of "ok gotta get through these A levels and I'm free" then don't.