What does it mean when someone say "force to be reckon with"

It seems like the British love to use this confusing word (reckon) on close to every setence.

So what does it actually mean?

"I reckon this and that and blah"
A force to be reckoned with = a significant force. 'To be reckoned with' is a set phrase meaning 'significant, powerful, challenging'.

I reckon = I think, I estimate.