It stands for _subject to_ and is the constraint on the optimization.
The problem says thay you should minimize $t_i$ with the constraint that $s_i^t$ is zero and $t=\lambda_{ij}^k$ for all $s_i^t$ in $\Gamma_{ij}$ and $k$ in $2,\ldots,n_{ij}$.
It stands for _subject to_ and is the constraint on the optimization.
The problem says thay you should minimize $t_i$ with the constraint that $s_i^t$ is zero and $t=\lambda_{ij}^k$ for all $s_i^t$ in $\Gamma_{ij}$ and $k$ in $2,\ldots,n_{ij}$.