Michael, the solstice is an astronomic event, that divides the seasons. During summer/winter, the sun moves from south to north, and vice-versa. (of course, watching it from the Earth :P). The solstice is when we end 'getting out of autumn' and start 'going in spring' (and vice-versa). So it's a period that the Sun 'stands still' on the sky.
Here, take a look:
If you read this line, remember not the hand that wrote it
Remember only the verse, songmaker`s cry, the one without tears
For I`ve given this its strength and it has become my only strength.