Hiya Venu
It is a colloquial expression.
If you have not seen someone for a while and speak or meet with them, you can find out what has happened to them or what they have been doing since you last spoke to them.
You are 'catching up' with their life story.
'A bit of a catch-up' would be a conversation, in person or by phone, to bring yourself up to date with what has been happening in each others lives.
Hope this helps
Gavin