Sure, so {{foo}} is the field itself, and the {{#foo}} {{/foo}} around it is a conditional block. So if foo isn’t filled out, nothing between {{#foo}} {{/foo}} will render.
So if you did
My name is {{foo}}
and foo wasn’t defined, you’d get “My name is” in a post
but if you use
{{#foo}}My name is {{foo}}{{/foo}}
and foo is not defined, “My name is” will also be excluded from the post