In Mandarin: 说 to speak, to say. 讲 to speak, to say, to tell, to explain( in detail) Eg: 讲课(to give a lesson),讲故事(to tell a story)
In Cantoness: because 说shuo, sh is the difficult pronunciation, so South Chinese( not only Cantoness) try to avoid this sound as well as: 'zh' and 'ch'. So, 讲 can replace 说 here.