Deck building or Pool building is a game mechanic for a card or board game where each player usually start the game with predetermined starter deck of cards. And when you play, there is usually a market row where new cards from a larger deck are displayed. You may add one or more of these cards to your deck and you usually have to purchase them by using a game currency.
So as you play, you add more cards that have many abilities and try to use these cards to defeat your opponent or opponents. You want to make an efficient deck.
Games like Dominion, Star Realms, Hero Realms, Dragonfire, and Clank uses the deck building mechanic.
Strategies
In some deck-building games, you want your deck to be efficient. A way to do this is to remove the not so useful starter cards from your deck as your deck grows. This is because in a deck building game, you can only play a limited amount of cards per turn. You don't want to be bogged down by starter cards that give you minimum currency or attack which denies you from using a more useful and powerful card in your deck.
In game Star Realms, there is a faction called the Machine Cult and in Hero Realms there is Necros faction which has cards with abilities where you can remove cards from your hand or discard pile from the game. You want to take advantage of that as your deck becomes leaner and brings out more of the powerful cards you purchased more often and less of the starter cards.
Another good strategy is trying to purchase cards that let you draw more cards. Since in a deck-building game, you can only play a certain number of cards in a turn, being able to draw an additional or more will definitely help you. Some cards may require a condition to draw a card like playing another card within the same faction that will activate the ability to draw an additional card.