A name sniper is a person who registers a new account (usually an alternate account) for the purpose of taking a clever username, a special user id (such as #7777777), claim the first account of a certain time period -usually the beginning of a year-, and/or other reasons.
Examples of Name Snipers
TheUsernameReserver (and the accounts in his friends list) | Justin_Bieber | Barack_Obama | CatsOfDuty | Poke_Min | P1NEAPPLE_Pen | User_Name | Name_Snipe | New_York | Chicken_Nuggets | Google_Pixel | Viritual_Reality | D_J | Sonic_Hedgehog | Good_Game | Game_Over | Iron_Man | Ban_Jo | Lucky_Clover | One_Direction | Miley_Cyrus | Dominus_Infurnus | Twitch_TV | Light_Years | Perm_Banned | Roast Chicken | Light_Bulb | Wii_Sports | Wild_Card | Serial_Number | Black_Canvas | Masters_Degree | Google_Account | Scrambled_Eggs | Valentines_Day | Jail_Cell | Role_Play | Foot_Step | Google_Support | Google_Tech | Head_Light | Smooth_Jazz | Fifty_Cent | Base_Plate | Dragon_Fly | Tri_Cycle | Bug_Squasher | Dragon_Flies | Stick_Bug | Ma_Ma | Bullet_Ant | Rice_Crispies | Black_Friday | The_ElderScrolls | Magic_Ball | Infinite_Warfare | Jason_Derulo | Force_Field | Chocolate_Milk | Black_Market | Adverb | Unicyclist_Frog | Wi_Fi | Hot_Chocolate | Death_Run | Black_Ops3 | Google_Translate | Peter_Griffin | Nailed_It | Stan_Lee | i_Pod | Front_Flip | Windows_XP | Chicago | Rick_Rolled | Account_Deleted | FaZe_Clan | Roblox_Talk | Krazy | Big | Wicked | Sand | Whoa | Skate | 4_J [Like 4J Studios, Which Made Minecraft] | H_Q | D_R | Uncutted | Previewer | NintendoSwitch | NotPleased | IFeelLost | Millionaires | Slurp_Ing | FIN1SH | Dank_Doritos | Samsung_Account | iTunes_Account | Seal_Egg | Christopher_Columbus | Voice_Actor | Broad_Sword | Insufficent_Fund | Counting_Down | Battlefield_Four | Chemical_Hazard | Undercooked_Meat | 1St_Class | Treasure_Map | NoCopyright_Sound | Incognito_Tab | Earliest_Bird | Dry_Skin | YT_Videos | Lolly_Pop | Living_Room | Water_Color | Tree_Trunk | Notice | DarthVader| all these usernames are in the group "Underscore Namesnipes".
Trackers/Lists
Lists are when a user makes an account with a name like "ExploiterList" and use the description to name different users of that type.
Trackers are usually "Admin Trackers" where users list every one of the specified social category.
On the rare occasion, you could message them and they'll add your requested user.
Sock Puppets
Sock puppets are when a user makes a parody of a user (Let's say his name is lolcow44, due to the font, a user could make IoIcow44, same but with i's). I_Nooby_Noob is taken, so someone might make I_N00by_N00b.
Name sniping was popular when ROBLOX started becoming public, that's why there are thousands of inactive accounts with clever names such as pizza, Place, and frank the tank, or any lucky IDs. Roblox refuses to reform them, in case they remember their password ten years later.