Our friends on Stack Overflow have a nice answer to thisnice answer to this. My favourite is the second answer, including the quote:
"Dependency Injection" is a 25-dollar term for a 5-cent concept. (...) Dependency injection means giving an object its instance variables. (...).
from James Shore's blog. Of course, there are more complex versions / patterns layered on top of this, but it is enough to basically understand what is going on. Instead of an object creating its own instance variables, they are passed in from outside.