For many early Quakers, the purpose of their life was to live with the same conviction as the first Christians, preparing for the imminent return of Christ, and all else was considered a distraction. So they dressed plainly, they did not indulge in dancing, and they were against displays of cut flowers.
The Testimony of Simplicity arises out of the desire to lead a simple life in order to focus on what is important: one's 'inner condition' instead of outward appearance. It also changes the focus away from attending only to one's own needs, towards recognising the needs of other people as well. Swanky clothes, high fallutin' speech, and conspicuous consumption are all intended to aggrandise oneself, which both lacks humility, and distracts from listening to the 'still, small voice of God' within.
Like much about the Quaker life, the Testimony of Simplicity has changed through time. There were periods when the Quaker style of dress was out of step with contemporary fashion, a fact long played on by the Quaker Oats Company in the United States (which has never had any Quaker involvement, but benefits from the image of wholesomeness and purity portrayed by the man in 'Quaker' clothing). It has, however, long been the case that Quakers have dressed themselves unostentatiously in the garb of the day. Similarly, the appearance of cut flowers on the Meeting Room table now lends an air of simplicity, when once it would have spoken of showiness.
Much is made of how Quakers used to address each other as well as other people, using the less-formal forms of 'thee' and 'thou' in place of the more reverential 'you'. Honorific titles were not used, not least because we are all 'equal in the eyes of God'. A common practice among British Quakers that can still be seen today is to refer to another Friend by their first and family names together (for example, Paul Parker, rather than Mr Parker, is the Recording Clerk of Britain Yearly Meeting). Older Members of the Society still address each other as "Friend".
Quakers also have a tradition of plain speaking', instead of puffing things up in flowery language. Examining seventeenth century English, including that used by Quakers, shows it to be highly verbose by modern standards: we all speak more simply now than our seventeenth century counterparts. Society at large has adopted a style of speech and writing to which seventeenth century Quakers were able only to aspire. Quaker writing today typically tries to avoid making claims that cannot be substantiated, or inferences that would be misleading.
More recently, the Testimony of Simplicity has focused much on discouraging unnecessary material indulgence and consumption, not least because the planet is unable to sustain the rate of consumption typical of western economies: not taking more than one needs, not always having to upgrade to the latest, the fastest, the zingiest. The Testimony has come to stand for leading a simpler life, and looking for ways to simplify one's life.
The testimony also discourages too much busyness so that adequate time is set aside for spiritual reflection.