Tipping is actively rolling the feet from one side to the other to make the skis roll on edge8 - the first primary movement in the essentials framework, driving the focus on the specific movement of tipping the inside foot.
Most skiers are "outside foot dominant", meaning they focus more on tipping the outside foot (eversion), while a few are "inside foot dominant", meaning there is a stronger action of and more focus on tipping the inside foot (inversion).
Tipping the inside foot is more important2, to activate the kinetic chain from the bottom up, maintain parallel shins, have the skis at the same angle and, more importantly, through the the phantom effect, it will also cause the outside ski to go on edge as well, especially at low speeds.
Many racers and high performance skiers have a keen focus on the inside foot - which is often lifted and tipped strongly into turns and this focus on the inside foot is now more and more a part of performance skiing, especially carving, at the higher levels.
In the first frame in the following montage, you can see the skier is late to tip the old outside foot (or rather untip it), for various reasons. In the frames 3-6, we can see the tipping into the new turn. Notice the active inside foot in frames 3-5 and parallel shins - for contrast, from frame 6, the skier gets lazy with tipping instead of tipping continuously throughout the turn:
See Skiing - a philosophy of movement for a detailed comparison of the different movement frameworks.
The focus on the inside foot and specifically tipping, was introduced to me by way of the Essentials of Skiing book (which is still the best way to spend 25$ on skiing).