What does "禦姐" mean?
It's a loan word from Japanese Kanji word "御姉".

It's wildly used in ACG subculture. ACG is short for Anime (not animation), Comics (Manga actually) and Games. Now it has become ACGN with the addition of "Light Novel" which holds similar market niche in Asia like Twilight does in western.

Anyway, in those subculture, popular characteristics of "ideal" image of Asian girls are cuteness and petiteness. "御姉", in contrary, is used to describe females that can be characterized as sexy, mature.

To gain a more vivid impression, you can google pictures for 御姐/御姉 and compare the result with the result of 蘿莉/ロリ.

!! Warning !!

I just know these stuff based on my intellectual curiosity. Don't blame me if people call you a pervert if you keep saying these stuff to them.