Mountain bike gears consist of two sets of gears, the front derailleur, and the rear derailleur. Changing gears involves shifting these gears to adjust to the terrain and conditions. To shift gears, you need to move the gear levers on the handlebars. The left shifter controls the front derailleur and moves the chain between the three chainrings. The right shifter controls the rear derailleur and moves the chain across the cassette, which houses multiple gears. Shifting to a lower gear will make pedaling easier, useful for climbing uphill, while shifting to a higher gear will make pedaling harder but faster, useful for flat or downhill terrain.