Raiding?
What is this, and can I join in?
It's a 6 player endgame activity. Depending on the team setup, you don't even need 6 players for two of the raids.
The game has 3 raids (vanilla version has 1 and the DLCs added two more to the game)
Each raid requires you to be at a minimum level for entry (for the taken king raid, it's based on your light level)
Vault of Glass or VoG as we call it is the original raid. Game suggests you be level 26 for the normal version of the raid. You can go in if you're at least level 22. The Hard Mode version suggests you be 30. You can go in at 26.
Crota's End (the dark below DLC raid) suggests players be 30 and 32 for normal and hard mode respectively. But you can be 4 levels below the suggested levels for entry into the raid.
Only the Taken King DLC raid checks your Light Level. I personally don't know what's the minimum level for either modes. I just know the suggested levels for them. You need to be level 40 and above 240 (I think) Light Level for normal mode. I can be wrong on that. Like I said, I don't know the minimum light level for this raid.
What level are you? If you meet the requirements, for normal mode then you're in.