In any case they are small posies of flowers. Ladies would hold them up to the nose or wear in their hair or bodices.
Violets, Lavender or any sweet smelling flower would be popular. Back in the Tudor times Oranges with cloves in them did the same thing, and although as far as I know, not called nosegays they accomplished the same thing. Maybe more popular with men than flowers? I believe they were pomanders.
In Victorian times the nosegay was given to ladies by sweethearts and often carried a message within the flowers so the men had to be careful of what flower they chose.
The language of flowers......there is I am sure a book that you can buy if you should want to know all about that. Try Amazon.com
Meanwhile you can get a short list here.
Naturally thats one reason Yellow roses are safe for a man to buy a woman because they signify friendship. They are also very beautiful.
So we maybe dont do such practical or romantic things today but the remnants lie in the Bridal Boquets and the corsage that you (Americans) may have recieved for the prom.
Then again, many young girls or even boys would pick a posy of Violets and take them home to mum.