Stem-changing verbs in the preterit

[Jessica]

*Verbs whose infinitive ends in -gar, like pagar, jugar, and llegar, end in -gué in the yo form of the preterite.

--¿Cuándo llegaste al cine anoche?
--Llegué a las siete y media.

*Verbs whose infinitve ends in -car, like buscar, tocar, and sacar, end in -qué in the yo form of the preterite.

--¿Qué buscaste en la tienda ayer?
--Busqué un suéter rojo.

*Verbs that have a stem change in the present do not have a stem change in the preterite.

Generalmente el supermercado cierra a las diez, pero anoche cerró a las ocho.

I don't get this one. What does it mean by "have a stem change in the present but do not have a stem change in the preterite"?