Challenge Rating
Challenge Rating was a mechanic introduced in Dungeons & Dragons 3e to try and make it easier for DMs to judge what kind of monsters were suitable to throw against their party and give them a reasonable chance of succeeding. It... didn't exactly work as planned. On paper, a monster with a particular Challenge Rating is a challenge for four player characters whose level matches the number. The number is based on an average of Hit Dice, Armor Class, Spell Resistance, Damage Reduction, damage output and all other abilities it might have. While the concept seems simple enough, in practice it means that there are low-level monsters with very HD or AC, can hit really hard or have abilities that either inflict a continuous effect that a party of that level cannot easily deal with or outright kills you.
An example would be the Skeletons from 5e. Regular skeletons are CR 1/4, meaning that four are a fitting match for a 1st level party. They have an AC that's pretty easy to deal with, don't have too many HP, don't deal a lot of damage (1d6+2 is perfectly survivable at first level) and have two immunities that a first-level party won't use a lot and a weakness that they are likely to have. The skeletal warhorse on the next page over however has a CR of 1/2, so two would be a match for a 1st level party. Except not only are they twice as fast they have double the HP and can deal a crippling 2d6+4 damage in melee, which at an average of 11 is a credible threat to even the toughest of Barbarians. It gets even worse with the minotaur skeleton on the same page: it's CR2 but has a whopping 67 HP, 40ft movement, and has a charge attack that increases the damage of its gore attack to a staggering 4d8+4 damage for an average of 22 damage. You'll need to be quite the badass to survive that. And that's not even taking into consideration the more powerful monsters.
Because of this correctly utilizing CR to create balanced encounters that don't stomp your players is more of an art than a science, which ironically is the same problem that WotC tried to solve with the concept of the Challenge Rating.