I'll try to give you a thorough explanation.

First, we have to define the word theta. Theta in Scientology is named after the Greek letter Theta and they use this symbol often. It is used to represent life force, or the êlan vital of an individual. Whether this life force comes from God, or some non-physical property doesn't matter, it just represents this life force that living things seem to have.

Next, we define MEST, which is coined after Matter/Energy/Space/Time, which represents basically everything within the physical universe, things you can touch, taste, see, observe, etc. Anything that is made up of these components in some fashion is regarded as MEST.

Finally, we come to the word entheta, which is short for enturbulated theta. This refers to the phenomenon of theta (life force) getting caught up in MEST but usually in disastrous ways, for example a car crash would be a good example of entheta - a lot of life force playing with cars and destruction of property, and upset of the individuals. (The opposite of this could be a painting, a good representation of theta interacting with MEST.)

So, if someone is enturbulated, they have some sort of MEST going on that they are caught up in or dealing with that can be upsetting. (Car crash, breakup of relationship, Timmy fell down a well, etc.)

Hope that clears it up.