Can someone explain to me why a "second wave" of the virus will be worse/has been worse in other countries? Wouldn& #39;t it be easier to impose restrictions when/if numbers start to pick up since we& #39;ve already been through it once?

I& #39;m sure there& #39;s something really obvious I& #39;m missing. I keep seeing tweets about how we shouldn& #39;t lift any restrictions because the second wave will be much worse and I& #39;m not sure where that& #39;s coming from.