A cost baseline is a snapshot of the expected costs throughout the project, in other words, a cost baseline is a time-phased budget.
I think the definition is still not clear, let me explain through an example. In every project schedule, you can view at any moment during the project what the expected costs are, for example, in a construction project, let's say that according to your schedule, that by week 15 of the project (when the excavation is done), the cost so far should be $1 million, but in reality, the project has costed you more, so when you compare your actual costs to the cost baseline, you will know if your project is now over budget, on budget, or under budget (this is very similar to calculating the Cost Performance Index, or CPI).
Note that if you introduce changes to your project, then your cost baseline will change, and you will need to take another snapshot to compare the actual cost to the planned cost.
The cost baseline is actually a subset of the project baseline (there are 3 project baselines: cost baseline, schedule baseline, and scope baseline).