According to Wikipedia:
Rifling refers to helical grooves in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis. This spin serves to gyroscopically stabilize the projectile, improving its aerodynamic stability and accuracy.
Rifling is often described by its twist rate, which indicates the distance the rifling takes to complete one full revolution, such as “1 turn in 10 inches” (1:10 inches), or “1 turn in 254 mm” (1:254 mm). A shorter distance indicates a “faster” twist, meaning that for a given velocity the projectile will be rotating at a higher spin rate.
In layman’s terms, the lands are the raised spirals inside the barrel of your AR. The grooves are the “ridges”.
When the bullet travels down the barrel after being fired, it uses the lands and grooves to create spin (twist referred to earlier), which affects accuracy, distance, and trajectory.