"Metroidvania" is a term used to describe a sort of exploratory 2D platforming game. It comes from the original games of the Metroid series and some of the middle-era of Castlevania games (most notably "Symphony of the Night", which is what primarily gave birth to the term).
The most defining feature is usually the world. Rather than have set stages or a world map, the entire game takes place inside one giant map, which you must traverse about. Not just any map, but typically a very complex map filled with all kinds of secrets. The map is typically divided into sectors, more in an aesthetic fashion to denote that the denizens and dangers will be different. Sometimes you have teleports to assist you in travel, but other times you just have to remember where to go in the map.
There's various elements at play in the exploration. There's typically tidbits hidden in spots, or puzzles that have to be returned to when you find appropriate equipment. The exploration is also non-linear - your limits of progression through the map are by various tools or powerups that you need to acquire, but the order in which you must acquire some of these tools is not set. And in many choices, there are many paths to get those tools, or even get past obstacles without those tools.
A very popular element of these games, especially in the Metroid franchise, is the concept of "sequence breaking". Using special tricks or otherwise mastering the utility of your basic abilities, you can sometimes access items and areas far earlier than the game expects you to.
As far as I know, Metroidvania games have always been 2D platforming games. Aside from the elements of exploration, the actual gameplay of Metroidvania games vary widely.
I'm not sure what the first game to do this was, but a "tell" is a private message sent to another player. I think it originates way back from text-based MUDs where to message someone you'd type something like tell <name> <message>.
These days many multiplayer games call those messages "whispers" instead, but often /tell will be aliased to it. For example, in World of Warcraft you can message someone either via /w for "whisper" or /t for "tell".
So, "please send tell" basically means "contact me privately to respond".
Best Answer
Soulbound is the system that RotMG uses for certain rare drops, usually from the most powerful mobs in the game. In RotMG, it simply means that only the person it dropped for can loot it. That means, if you saw it on the ground, it was yours - no one else could pick it up. Only some of the best items in the game are protected that way, so it's probably pretty valuable.
I don't play enough RotMG to know what that exact item does, sorry. After being looted, soulbound doesn't actually do anything else that I know of - you can trade it or drop it, or lose it to death like any other item. It just keeps the soulbound "tag" to remind you that it was soulbound on drop.