What do the two red stripes at each end of the Canadian flag represent?

THE Pacific and Atlantic oceans
[From ultimateflags.com 2003 article, no longer online]

The red in the flag is representative of England and the white represents France. The pattern was derived from that of the Royal Military College of Canada.

https://en.wikipedia.org/wiki/National_colours_of_Canada

Prime Minister Pearson had previously favoured a similar design with a sprig of three maple leaves on white, between two vertical blue bars, with those blue bars explicitly representing the Pacific and Atlantic oceans (Canada's coat of arms includes the Latin motto "A mari usque ad mare," translated as "From sea to sea"). The design that was selected, with the single red maple leaf on white between two red bars, was seeking simplicity and distinctiveness, and its designer (Dr. George Stanley)stated there was no symbolism behind it.

The use of two thirds of red has no particular significance.

This is laid out in a memo on the subject at http://people.stfx.ca/lstanley/stanley/flagmemo2.htm His sketched design in this memo was later modified to widen the white middle section to twice the width of the red bars (now officially called a Canadian pale in heraldry). See also http://www.thecanadianencyclopedia.ca/en/article/the-stanley-flag/

Stanley's memo does discuss Canada's national colours being red and white, with red considered to represent the English and white the French. He also acknowledges in it that his design inspiration as coming from the Royal Military College's flag.