what does 99 10 mean? Thank you

what does 99$_{10 }$mean? Thank you

$\begin{array}{lcl} \text{Decimal Numeral System - Base-10} \\ \text{Decimal numbers uses digits from 0..9. }\\ \text{These are the regular numbers that we use } \\ 99_{\mathbf{10}} = 9\cdot \mathbf{ 10} + 9 \\ \text{Example:}\\ 2538_{ \mathbf{ 10}} = 2\cdot \mathbf{ 10}^3+5\cdot \mathbf{ 10}^2+3\cdot \mathbf{ 10}^1+8\cdot \mathbf{ 10}^0 \end{array}$