What is the tight side?

ok, I need somebody to clarify for me. In the video about joining two strands together by just doing several stitches with both strands, Amy talks about knitting into the tight side of the stitch. What is the “tight side?”

Without looking at the video, I guessed that the tight side was through the back loop, but now after I watched it, it’s on the tight side as in “a bit tightly”