clarification regarding an if statement in ksh

Im quite new to ksh scripting, please mind me if this is too obvious

What does this mean in ksh? Is this also a way to write an if condition?

[[ -n $MSRV_SGL ]] && { msrv_ps || return 1 }

msrv_ps is a function

How does this read? if the length of string $MSRV_SGL is non zero....?

I havent come across such expressions in any online examples.