Until the 16th century, Summer Solstice meant just one thing in Britain and most of Europe - fires! It was a tradition which could be traced back with confidence over eight and a half centuries. More tentatively, there's an argument that these fires had been lit every Midsummer for fifteen hundred years.
Unfortunately, they had picked up some Catholic influences over the 13th-16th centuries, which was the excuse that the Protestants of the Reformation used to stamp them out.
However, the Midsummer Fires died out slowly. As late as the early 20th century some isolated rural areas still lit them. Then Cornish nationalists picked up the tradition, reviving it as part of regaining their heritage.
The baton got passed to the Pagans, who started to become more public with their Summer Solstice Fires in the second half of the 20th century. In short, they were never really extinguished completely and they are enjoying a resurgence now. (For the full story, may I recommend Ronald Hutton's The Stations of the Sun.)
But what did the fires all mean? In a superficial sense, they were an excuse for a great party. Around the bonfires and wake fires, feasts were shared and feuds were forgotten. It was a great show of communities pulling together and relaxing in a spirit of goodwill.
In a deeper sense, it was all about protection and blessing. Basking in the sun - in all that heat and daylight - people were healthier than they may have been in the cold of winter. Food was more plentiful. This was as strong as they would be all year (a situation which has been altered for modern Western communities, with the advent of electric light, heaters and 24 hour supermarkets).
But humanity isn't the only species to benefit from nature's bounty in this way. Insects were also amassing and they can carry disease. Also the crops were about to become more vulnerable, as worsening weather had the potential to destroy them on the eve of harvest.
What people needed from Midsummer onwards was a great deal of luck and divine intervention. The fires undoubtedly honored the sun above, at the zenith of its strength.
As torches were carried around fields, burning branches wafted over and under cattle, and individuals leapt over the wake fires, it was all with the same purpose in mind. They were seeking some of the sun's protection and blessing for the dark times ahead.
Even the feeling of goodwill could play into this. If you have made a friend of your avowed enemy at the Midsummer fires, then you won't have to watch your back with them in winter. If you've been generous with your neighbors during the party, then your kindness may be repaid when the earth freezes or illness plagues your family.