The "subscriptions" switch enables options via which people can subscribe to email notifications of updates on your site. This includes subscribing to forums, subscribing to be notified of all new topics and subscribing to new posts in a discussion thread. Note that it doesn't include the notifications of new private messages -- the profile option to receive those will be available as long as private messaging is on.

Beneath the subscriptions switch is the sub-option for keyword subscriptions. Keyword subscriptions allow users to write a list of keywords in their edit profile page to request to be notified of any new topics that match those keywords. It's essentially a watch list so they can be aware with something matching their interests appears. Keyword subscriptions will slow your site somewhat.

On a site with a lot of members and a lot of daily submissions, the volume of email will begin to slow down your web server. To solve this, you can turn off the "instant subscriptions" subswitch. Instant subscriptions send every subscriber an email the moment something is added, so if there are hundreds of additions in a day that can be hundreds of times more emails than a daily digest or weekly digest subscription. The digests also run more efficiently over time, while the instant subscriptions could cause a submit to process very slowly or fail to an error as it attempts to calculate who needs an email all at once. If you're having problems with slow submissions or overall slow site on a site with thousands of members and many submissions per day, try switching off the instant subscriptions and see if it helps.