$\mathrm{MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2}$ The reaction given above is a redox reaction because in this case :

In a redox reaction, oxidation and reduction occur simultaneously. Oxidation is defined as the gain of oxygen or the loss of hydrogen. Reduction is defined as the loss of oxygen or the gain of hydrogen.

1. In the reaction $MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2$, $MnO_2$ loses oxygen to form $MnCl_2$. Therefore, $MnO_2$ is reduced.
2. $HCl$ loses hydrogen to form $Cl_2$. Therefore, $HCl$ is oxidised.

Since both processes occur, it is a redox reaction where $MnO_2$ acts as the oxidising agent and $HCl$ acts as the reducing agent.