Soilder is a common misspelling of
soldier, usually meaning whoever has typed the message containing soilder is a small child who has not
yet learned the difference between the ground and a
soldier, and can often be found being used by
new players in the game Roblox.
See Solider for similar misspelling.
For example, that noob who keeps misspelling literally everything he types might say, "how to bea n us mairnes soilder?". Everyone will continue to ignore him until some hopeless soul tries to explain it to him, "so you
go into menu and select... (he will explain all of it to him)". Of course, the noob who keeps misspelling literally everything he types will not understand, and ask the same question until he either leaves or gets fed up with nobody answering him.