At the beginning of the GPLv3 terminology used is explained:
To “propagate” a work means to do
anything with it that, without
permission, would make you directly or
secondarily liable for infringement
under applicable copyright law, except
executing it on a computer or
modifying a private copy. Propagation
includes copying, distribution (with
or without modification), making
available to the public, and in some
countries other activities as well.
To
“convey” a work means any kind of
propagation that enables other parties
to make or receive copies. Mere
interaction with a user through a
computer network, with no transfer of
a copy, is not conveying.
In any case, your phrase is in the section about patents. And it means if you distribute code under GPLv3 and grant for some users a patent licence, you grant this patent licence for all users of the code. propagate by procuring conveyance includes the possibility of an additional contract that is part of the transfer of a copy to a party.
GPLv3 does not put any additional requirements on SaaS Vendors. What you are talking about is the Affero GPL. It requires an SaaS provider to comply with all the requirements of the GPL not only when conveying software, put also when providing services through the software. This means if you i.e. make changes to an Afferp GPL licenced software and provide a service with this software, you have to make those changes public under the same licence, in the same way someone who distributes a GPL licenced software would have to do it.
DISCLAIMER: Nothing written here constitutes legal advice, but its sole purpose is an academic discussion about an issue of law. If anybody needs legal advice concerning their particular situation, they should consult a licenced attorney in their jurisdiction.