Exothermic reaction.
Heat is written on the product side, which means heat is given out when hydrochloric acid neutralises sodium hydroxide:
\[\mathrm{HCl + NaOH \rightarrow NaCl + H_2O + heat}\]
A reaction in which heat is released is called an exothermic reaction. You can feel the beaker becoming warm when an acid and a base are mixed.