"Metabits are prestige currency. Your entropy and idea production creates metabits. Gain more entropy and idea to produce more metabits. Each metabit will require more entropy and ideas than the last to earn. If your simulation starts to feel slow, restart the simulation." ― Official description in the Reality Engine
MetaBits are the prestige currency of Cell to Singularity and are used to upgrade the Reality Engine that runs the simulation. You gain unclaimed MetaBits by producing Ideas and Entropy, which are then claimed and made usable by "rebooting" the simulation. This ability becomes available after buying the first Singularity.
The total amount of MetaBits earned is the "simulation level," which passively increases the production multiplier by itself. This is independent of the amount of MetaBits on hand, which can be spent in the Reality Engine tree to increase production, click value, or offline production value, you can also get MetaBits in the main simulation and the Mesozoic valley and most useful, in the cube tree in the reality engine and you can buy the Relativity upgrade by buying 50 middle ages and you can get a lot of MetaBits
The formula for MetaBits gained after a reboot is: (Entropy + Ideas)^(1/3) / 10000 - 1.