The initials RMS stood for Royal Mail Steamer, a ship which carried mail (as well as passengers). A steamer was a ship powered by steam engines, which used coal as a fuel. This produced lots of smoke which you can see coming out of the funnel.
On the outbreak of World War 1, RMS Laurentic was converted into a troopship, and brought thousands of Canadian soldiers to the UK.
In January 1917 she was sunk by a mine off Ireland (which had been laid by a German U-boat) and sank within an hour with the loss of 354 lives, including many Canadians.
The loss was keenly felt, not least in Folkestone where large numbers of Canadian soldiers were based at Shorncliffe Barracks. It is likely some of them had travelled to the UK on this ship and may have known people on board.
Also lost in the disaster was Chief Petty Officer Charles Albert Taylor, age 24, a member of the Royal Navy crew, who lived in Folkestone.