A walking wild is a wild that shifts position on each spin instead of disappearing. Most move one reel to the left per spin, triggering a free respin each time, until they walk off the reels.
The effect is a short run of spins with a guaranteed wild that changes position each time. A wild landing on the rightmost reel can therefore give several respins, one for each reel it crosses. Some games move wilds in other directions or randomly, and some combine walking with sticky or expanding behaviour.
The extra spins are part of the feature, priced into the game's RTP rather than added on top. See the walking wilds feature page.