Epoch time is defined as "the number of seconds that have elapsed since January 1, 1970 (midnight UTC/GMT), not counting leap seconds (in ISO 8601: 1970-01-01T00:00:00Z). Literally speaking the epoch is Unix time 0 (midnight 1/1/1970), but 'epoch' is often used as a synonym for 'Unix time'.
So, if you have a number of records containing epoch/unix times, you can use a DATEADD() function in a formula field to convert that to a more friendly date by adding however many seconds (as specified by your epoch time string) to 1/1/1970.
Here's an example using the formula below:
You can use a similar formula to convert the times in your base!