align-items and align-content both deal with cross-axis distribution. align-content only does anything when working with multiple lines of flex-items; it will distribute the space between and around each line. When using align-content: stretch, it will stretch the lines to fit the container. align-content seems to kick in when there's 1 line of flex-items; it will stretch these along the cross-axis to fit their container as well.
justify-content deals with the main axis distribution. As far as I know, there is no stretch option here. We use flex-grow and flex-shrink to handle how space is distributed along the main axis.