Tournaments are about stack preservation
One of the most important concepts in tournament poker is stack preservation. When your stack begins to dwindle, all of the hands you get in the future are actually worth less. If we were to get a hand like pocket aces and double up with them, we'd rather have our 5,000 chip starting stack than have called several bets and let our stack fall to 3,500. In the first case, we'd have 10,000 chips. In the second, just 7,000. That amounts to a significant chip difference, as tournaments require us to double up five or ten times in order to win the game. It's important in an MTT to have a stack that's as big as the other stacks at the table, and stack preservation goes a long way to ensuring this is the case.
What does stack preservation mean? Simply, don't risk chips that won't earn you a return, and prefer a high-frequency return over a high-value return. For example, if you have the nuts on the river, and you have the option to make a small bet that will be called more often or a large bet that will be called less often, it's often more valuable to make the small bet. It also means not calling before the flop, but raising or folding instead. Calling before the flop, especially shorter than 100BB deep, leaks chips. At the early levels of small stakes tournaments, you often see players calling before the flop. Don't fall into the trap of saying "everyone else has called, so I have odds to call." In higher stakes tournaments, or at the final table of medium-stakes tournaments, you see very little calling before the flop. Players who make this mistake go broke.
Master the pre-flop and flop game
In poker tournaments, the hand often ends before the flop. If it doesn't end before the flop, it often ends after the flop. Very rarely do tournament hands go to the river. For this reason, we recommend mastering the early streets of poker play. Understanding continuation bets, understanding play between 15 and 30BB, and knowing when to steal blinds will go a long way to improving your winrate at poker tournaments.