To field in Glide with the column in your sheet which contains your email.
Glide gives you access to configuring these parts of the mailto link in the other fields, but they are optional:
Subject = sets the subject text of the email
Body = allows you to add to the main text of your email
CC = copy the email to another address
BCC = secretly copy the email to another address
If you wanted to utilise all of these, your sheet might look like this:
And after you'd bound all the columns to the right properties in Glide...
...the email link that people would be visiting would be: