A baker on the Tezos network is covered in full in the entry Baking (Tezos), which explains how blocks are produced, how rewards are earned and how delegation works. This page gives a short definition and explains the name.
A baker is a participant that runs Tezos node software with a stake of tez, the network's native coin, and is selected by the protocol to propose new blocks and attest blocks proposed by others. In other networks the same job would be done by validators or a block producer. Bakers earn rewards for this work and can be penalised if they misbehave, for example by signing two conflicting blocks. Holders of Tezos who do not want to run a node can delegate their coins to a baker and share in the rewards while keeping control of their funds.
The name comes from the Tezos community's choice of the word "baking" for block production, a playful alternative to "mining" on proof-of-work chains and "forging" or "minting" on some other proof-of-stake networks. A baker is the person or organisation that bakes, so the two terms describe the same activity from different angles.