a newbie; a person who has started something just recently
It’s often used as にわかファン, meaning “a bandwagon fan”. It’s usually used in a disapproving way to describe someone who pretends to know copious knowledge about something that they’ve learnt or got interested in recently.
In non-slang contexts, にわか is often used as “にわかに” meaning “suddenly”, or にわか雨 meaning “a rain/shower that starts falling suddenly”
During World Cup, lots of bandwagon football fans pop up and talk about football fervently.