Questions & Answers about Соседка сердится, когда дети кричат под её окном.
Сосе́д = neighbor (male neighbor, or neighbor in general if gender is unknown or irrelevant).
Сосе́дка = female neighbor.
The suffix -ка often makes a feminine noun from a masculine one:
- учени́к → учени́ца (pupil m. → pupil f.)
- вра́ч → (no regular feminine) but
- сосе́д → сосе́дка
In this sentence, the speaker either knows the neighbor is a woman or wants to emphasize that fact, so сосе́дка is used. In English, neighbor doesn’t show gender, but Russian usually does.