What is RG?

[BALAJI_CHENNAI86]

The following command adds the disks 5a.17, 5a.19, 5a.20, and 5a.26 to the rg1 RAID group of the aggr2 aggregate:

aggr add aggr2 -g rg1 -d 5a.17 5a.19 5a.20 5a.26

Please explain what is rg1 Raid group and why it needed?

[PNAGASUBRAMANYA]

basic physical unit is Disk

Disks are grouped into Raid Groups(RG) to ensure availability of Data in case of a disk failure. the raid type used will be RAID-DP

Aggregates contain Raid Groups

logical storage volumes (partitions) are carved and spaces allocated to users to store data

in your example RG1 is an existing raid group. Disks are being added to the RG RG1 to expand space in the Aggregate aggr2

[JGPSHNTAP]

I hate to be the bearer of bad news, but man, RG's are the underlying of all the disk layouts for netapp.  You need to understand this before you even touch a netapp, in my opinion..

Start here -

https://communities.netapp.com/docs/DOC-12850