"Top Decking" in Trading Card Games refers to drawing a card (whether through its draw phase or through a card effect) that gives the player, who normally is in a bad situation, a large amount of advantage or even allows it to win the game because of that card.
Drawing cards is perhaps the most common and a very important element in TCGs, as drawing a good/bad card at certain key moments can decide who wins the game. When a player who is seemingly about to lose draws a card that allows them to turn the game in their favor, it is considered a "Top Decking", as they most likely would've lost the game had they not drawn that specific card.
The more derrogatory version of the term, "Lucksack", can also be used to refer to this, as some can argue that the player only won because "they got lucky and got the card they needed the most".
In Future Card Buddyfight, the "Charge and Draw" system essentially gives the players two draws per turn, giving them two chances to "Top Deck" a useful card.
Top Decking can also refer to effects which are dependent on revealing a specific card, like for example, the effect of Brave Equipment, Glory Seeker and similar cards.
Note that using a card effect to manipulate the top card of a deck, for example by the effect of Fortune Select! (aptly named) and then using that card to turn the match into your favor is not considered top decking.