Hello @ankit.dhawan,
An explanation in easy terms is like this:
1. Tunnel-group: Is the one that handles the connection, your connection will land in a tunnel-group and in this part you configure the AAA and you can decide how to assign the IP for the connection.
2. Group-policy: Are the extra information you want for the connection, I.E. if you want to apply restrictions with an ACL, DNS servers config, DHCP configs, Protocols you want to work with, you can apply also IP addresses here.
In a tunnel-group you can have only one group-policy, but you can have one group-policy attached to different tunnel-groups. This means for every connection on the ASA you need one tunnel-group and one group-policy associated.
HTH
Gio
