A Monster Card's Level is a rough indication a card's power ranking, represented by a number of stars in the top-right. The term Level is sometimes informally called Stars, and even appears in some card errors. The highest Level a monster can have is 12, and the Minimum is 1. A Monster Card cannot be treated as Level 0, even with an effect, the minimum will always be Level 1.
Monster cards from Level 1 to Level 4 may be Normal Summoned from the Hand with no cost or restriction (unless the card itself has a restriction against it in its effects). Level 4 monsters are the strongest monsters that may be summoned from the hand in this way, and are thus the most commonly used monsters. Levels 5 and 6 monsters are stronger but either require a Tribute to be Normal Summoned or have a Special Summoning condition, and Level 7 and higher monsters require two Tributes to be Normal Summoned, assuming they may be Normal Summoned at all. Level 9 and higher monsters are relatively rare, and usually are stored in the Extra Deck rather than the Main Deck, though there are noteworthy exceptions.
A common rule is that level 3 and lower monsters generally have 1700 and under ATK, level 4 monsters have 1900 and under attack, level 5 and 6 monsters have 2500 and under attack and finally, level 7 and higher monsters have 3000 and under attack. Usually monsters who go break this rule have penalties or special conditions, although this trend is broken by a few cards.
Due to the higher requirements involved in summoning them, higher-level monsters are typically not as numerous as lower-Level monsters in a player's Deck, assuming the Deck is properly balanced.