In the conventional concept of how the passage of time operates, time is divided into three distinct regions; the "past", the "present", and the "future". The past is generally seen as being immutably fixed, and the future as undefined and nebulous. As time passes the current present becomes part of the past, and part of the future becomes the new present. In this way time is said to pass, with a distinct present moment "moving" forward into the future and leaving the past behind.
This model of time presents a number of difficult problems, both philosophically and in terms of current accepted scientific theories. For example, special relativity has shown that the concept of simultaneity is not universal, with different frames of reference having different perceptions of which events are in the future and which are in the past; there is no way to definitively identify a particular point in univeral time as "the present". Furthermore, there is no fundamental reason why a particular "present" should be more valid than any other; observers at any point in time will always consider themselves to be in the present. Even the concept of "time passing" can be considered to be internally inconsistent, by asking "how fast does time pass?"
Block time overcomes these various difficulties by considering all points in time to be equally valid frames of reference, equally "real" if one prefers. It does not do away with the concept of past and future, but instead considers them as directions rather than as a state of being; whether some point in time is in the future or past is entirely dependent on which frame of reference you are using as a basis for observing it.
Since an observer at any given point in time can only remember events that are in the past relative to him, and not events that are in the future relative to him, the subjective illusion of the passage of time is maintained. The asymmetry of remembering past events but not future ones, as well as other irreversible events that progress in only one temporal direction (such as the increase in entropy) gives rise to the arrow of time. In reality, there is no passage of time; the ticking of a clock measures durations between events much as the marks on a measuring tape measures distances between places.
Block time makes two assumptions, which are separable. One is that time is a full-fledged real dimension. The other is immutability. The latter is not a necessary consequence of the first. If random changes are possible, the result may be indistinguishable from the Many-Worlds Interpretation of quantum mechanics.