David,
Thank you for your message.
A hashtag is represented by the following symbol: #.
By adding a hashtag in the subject line, you can categorize  your message. For example, for this message,  we assigned the category of “#Help.” If your message was related to Polish research, you could add the hashtag of #Poland in the subject line. In this way, other researchers would know what category your post was related to.
I hope this is helpful. Please let me know if I can ever be of help.
Thanks for being part of the JewishGen family!
Avraham Groll
Executive Director
JewishGen.org