when you stand in a line behind two of your friends, you will be the

The completion of the phrase "when you stand in a line behind two of your friends, you will be the..." is that you will be the third person in the line. In other words, if you stand behind two friends in a line, you are the third person standing there. This is a straightforward positional statement indicating your place in the sequence or order within the line. No other special or figurative meaning appeared from the search results in the context of this phrase, so the literal positional meaning is the correct interpretation.