I have a question.. What does "Leeching" mean?

Only taking and never giving anything back.

Yup; it means to just enter in giveaways or expect people to give you games, codes, items, etc, when you show no productivity or effort in giving back or enhancing the community with games or contributory giveaways.

sucking someone blood (must be a worm)

Using leeches for bloodletting or medical therapy. They also call it hirudotherapy.

i was a leech but then i took a...

To sum up they are forum lurkers.(Not to be confused with forum posters.)

To drain (resources) without giving back.
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People that come to this site just to enter giveaways without doing anything else. But it's different from person person what "leeching" means, for some it's when someone doesn't give games away or when someone isn't active in the forum (and there are also many other reasons). There is no exact definition of it.

Downloading a torrent then not seeding back.

he's the wee guy who runs my local Chinese takeaway.