A gerund is a noun formed from a verb by adding the suffix "ing". The following are all gerunds:
climbing / polishing / eating
Although a gerund is a noun, it can still take an object (like a verb). The gerund, its object and all modifiers (adjectives and adverbs) are known as a gerund phrase.
Eating blackberries without washing them will make you ill.
I am not prepared to authorise climbing the cliffs in the dark.
Glossary of grammatical terms