Air flow - incompressible vs. compressible
This question should lead to taking a basic aerodynamics course or finding text books for that information on line. As a very simple, high-level over view the lift and drag characteristics of an airfoil change as Mach number approaches unity. At low speed, air flowing over an airplane is reasonably approximated as incompressible and scales with impact pressure that is proportional to the square of airspeed. As speed approaches Mach 1 the airflow experiences more and more compression yielding variation in the aerodynamic characteristics.
Airplanes are designed to optimize performance at a specific Mach number. To fly an airplane faster than its optimum cruise Mach number would require significantly more thrust as drag increases rapidly with higher Mach.
For most commercial transport airplanes the upper limit on operating velocity is airspeed at lower altitude and Mach number at high altitude. This gives rise to maximum operating velocity (Vmo) and maximum operating Mach number (Mmo). For any given airplane there will be an altitude at which Vmo and Mmo are reached simultaneously - this is called the Vmo/Mmo corner. Below that altitude Vmo is the operative airplane velocity limit. Above that altitude Mmo serves as the limit. The altitude of the Vmo/Mmo corner usually falls between 25K and 30K feet.
I hope this tiny intro encourages you to explore more understanding of these topics through course work and text books.