"Gerund," as a grammatical term, is used in many languages to identify different types of specific word usage.
In English, a gerund is a verb that is being used as a noun, usually describing an active process. Most English gerunds can be identified by their common ending suffix, which is "ing." Examples of gerunds in English could include "Singing is fun;" "Cleaning is hard work but necessary;" and so on.
English gerunds can serve as a verb within a noun phrase, with an adverb or an object added to the phrase. Consider the sentence "Playing the piano takes lots of practice." "Playing the piano" is a noun phrase, with "playing" used as a verb in combination with the object of "the piano"; the phrase as a whole is the subject of the sentence, in particular the gerund describing the activity being discussed.