A Multiplier increases the amount of points earned.
GoL and ToO
Guardian of Light
The more points a player collects the fuller his or her Relic Power Meter gets. When about half full the points scored for killing enemies doubles, and when full it triples.
Temple of Osiris
The amount of points the player gets for collecting Gems depends on the multiplier that is displayed on top of the Health Bar. There are 1x, 1.5x, 2x, 3x, and 4x multipliers. Additionally, the more players enter a Multiplayer game, the more points one gets for collecting gems. See Gems#Temple of Osiris for a complete list. The multiplier can be raised by collecting gems and killing enemies without being wounded in battle.
In Lara Croft: Relic Run the current multiplier is displayed in the top left corner. It increases the points earned for each meter Lara runs. It can be increased by collecting Coins, Clues, and Powerups or by shooting Enemies, Health and Coin Crates during combat sequences. On the other hand it decreases automatically after a certain time without Lara having collected any items. (Multiplier Decay)
At the beginning the base multiplier is 1x. Upgrading some of Lara's Outfits can increase the base multiplier; e.g. a completely upgraded Standard Outfit lets the player start each new run with a multiplier of 4x. The multiplier can not drop under this base even if Lara does not collect any new coin or other items.