What does the three horizontal lines of ash or vibhooti on Lord Shiva’s forehead indicate ?

Shiva, the ascetic applies ash on his entire body. This indicates that everything is ash to Him since the ultimate truth of mortal, the physical body is ash.
Shiva applies 3 horizontal lines of ash or Vibhooti on His forehead.

These three lines represent The 3 Gunas namely Rajas, Tamas and Sattva.

Shiva is the three-eyed God (Trinetra).

Shiva as the trikala darshi ( who knows past , present and future ).

Shiva is beyond the 3 karmas namely Sanchitta, Prarabdha and Agami.

And the devotees who do earnest prayers to Shiva too will be relieved from the 3 karmas , and thus attain Moksha/liberation.