Yes, each hosting account has a Shared SSL included and it can be used free of charge. For more information please check this link:

http://kb.siteground.com/article/How_to_use_the_free_shared_SSL.html

Siteground also offers Private SSL which is an additional upgrade. For more information please check this link:

http://kb.siteground.com/article/Why_do_I_need_a_private_SSL_certificate.html