These terms are commonly used in the world of P2P or FTP download.
The seeders refers to the users having the complete file(s) and sharing them, that is to say, the ones uploading the data.
Leechers refers to the users downloading the files. This term can also refer to a person who advocates the paternity of a link, without being the one who originally provided the file for download on the sharing platform.
Thus, the more seeders, the quicker the file download. Conversely, if there are only leechers, there is nothing to download.
Thanks to Hyunkel-on for this answer on the forum.