Interlacing Constraint
Each entry must be ≤ the entry above-left and ≥ the entry above-right. This creates a "sandwiched" constraint between adjacent rows.
About GT Patterns
Gelfand-Tsetlin patterns arise in representation theory of unitary groups GL(n). Each valid pattern corresponds to a basis vector in the irreducible representation indexed by the top row λ.
The number of valid patterns equals the dimension of the representation. They're also in bijection with semi-standard Young tableaux.