I've setup my router from an online website, it works great.

Can someone please explain what exactly the rule below purpose is ?

I thought it was for masquerading my clients ip address to public ip's
for browsing etc , but when I disable the rule my clients can already
connect to the internet.

Did the clients already have an active connection when you disabled the rule?

Without that masquerade entry, your clients shouldn't be able to access internet unless there is something else on the network (maybe the ISP modem) providing NAT or the clients have static IP addresses on the internet.

Did the clients already have an active connection when you disabled the rule?
.

Yes when i tried disabling the rule again i wasnt able to get a connection
from a client to the internet , maybe i was looking at some cached data
the first time.