Internet meme started in the Dungeons & Dragons Online MMORPG first and then refined (mostly in form) in the DDO Forums.
The definition is hard to quantify. Some would call it a "slam" or some sort of "snarky" reply. While that can be true it is much more subtle than that. It's meaning is more of a feeling than anything else. How can one possibly define Love adequately? They can't, of course, and likewise, no one can adequately define Hi Welcome
Form: Just to review the form quickly, it should always be written with a capital H and a capital W. No punctuation is permissable either. It should always be written - Hi Welcome and no other way. If you're writing it in another language (and assuming the language has capital letters), only the first letter of each word should be capitalized and again no punctuation is used. The lone exception to the "only the first latter in each word is capitalized" rule is for the Chinese form not written in characters. In this case Ni Hao HuanYing is correct. This is due to a peculiarity of the Chinese language.
Here is one example:
Someone (most likely a n00b or a known forum jackass) has just given a long forum rant displaying a lack of experience, social adeptness, and most of all...intelligence.
You reply with: Hi Welcome
In this particular case it may mean, "Allow me to welcome you both with my literal words and with an active display of how much you fit in by being tone deaf, dumb, and uncritical of your babbling myself. Because anyone able to engage your arguments would rip them to shreds, the most sincere welcome you can get is from someone unwilling to do so as demonstrated by their simple vocabulary and punctuation and seeming lack of thought put into the response."