A condition is, in its simplest form, a function that returns a True or False value. In processes, they are used to control the things that must be met before a process can be run (pre-condition).
A condition needs to be evaluated, so there may be some run-time context that is passed to the condition so that it can be evaluated. A condition could be one based on the value of a field. For example, you could have a condition such as "Is Legal Aid matter?" which could be computed at run-time, but could also be computed just once (when the matter is created), assuming that the field is static for the life of the matter. This is a "static" condition that changes rarely. Once met it remains met unless the process is reset. See Add pre-conditions to a process.
Note
Users with the relevant permissions can manually complete a process even if the pre-conditions have not been met.