Each year, the summer solstice signals the "beginning" of the summer season. The summer solstice is very similar in meaning to the winter solstice, but opposite in the position of the sun.
The summer solstice is the day during which the sun is at its most Northern point in the sky (usually near June 20th or 21st). Consequently, it is also (typically) the longest day of the year, having the shortest night. The word solstice itself comes from a Latin phrase meaning "the Sun stands still". For a few days before and after solstice, the sun would be changing declination so slowly that it would appear to be standing still.
The easiest way to visualize this is to imagine yourself standing on a sphere (the Earth). If you're standing in the Northern hemisphere, the sun will always appear to be slightly South of your location. The more you tilt the North pole away from the sun, the further South the sun seems to dip. The tilt of the Earth's axis is not changing, but its position relative to the Sun is, producing the same effect.
The summer solstice occurs on the following days in upcoming years: