The word solstice is formed from Latin sol, “sun”, and sistere, “to come to a stop, to stand still”.
So the Sun stands still at this time of year, either at the zenith or the nadir of the annual cycle, depending on the hemisphere in which you dwell. Either way, it is the point of « enantiodromia », the still point from which the return to the opposite commences: winter starts moving towards summer, and the days slowly grow; summer starts moving towards winter, and the nights inexorably increase. Together north and south form a dance, like a double spiral, expanding and contracting in relation to each other's activity.
If you're at the winter-point of this cycle, plant the seeds that determine the arc of your next six months. If you're at the summer-point of this cycle, you are at the culmination of seeds already cast, and your arc will begin its decline. Plan how you will harvest your fruits.
In terms of religious symbolism, it may be noted that Christ is traditionally born at the nadir of the year, in the midst of wintery death, signifying light emerging from darkness. In Spring, when the world shakes off Winter's shackles and comes to life, Christ is killed and overcomes death. Once again, it holds to the principle that each pole contains its opposite. Birth in the midst of death; death in the midst of life. “In the midst of winter”, remarks Albert Camus, "I discovered within me an invincible summer (Au milieu de l‘hiver, j'ai découvert en moi un invincible été).