"Net movement" is a way of describing the "final" or "total" amount of movement taking place, although neither of these terms are sufficient to truly describe things in the same way the term "net" does.
"Net" basically has three states: positive, negative, or no change. The exact number attached to these values is less relevant. Finding whether the net value is positive, negative, or neutral is the result of adding up all the factors that contribute to the system being evaluated in some way. A good example is working at a job.
Ostensibly we work at jobs to make money. One would hope that you end up with more money at the end of the day than you began with. A bad job would be one where you lose money; for example, what if you had to commute a very long distance, then pay money to park your car, to work for two hours at $5 an hour, and then drive back home. You would have to evaluate all the negative and positive "flow" of money in this job:
driving to work: -$ for gas and car maintenance
parking: -$ for the fee
Working at the job: +$ in payment
driving home: -$ for gas and car maintenance
So you would probably end up with a net loss of money.
In terms of biology, net movement would describe something like a cell, organism, or ecosystem, and it would evaluate things moving in (positive) and out (negative) of that system. For example, if you surround a cell with water, there will be some movement of water into the cell, and some movement out of the cell. The net movement would describe whether the TOTAL effect was for water to move in or out. A cell placed in pure water would have a net positive flow, because water would flow into the cell, and the value of this flow (in terms of molecules, etc) would be greater than the flow of molecules out of the cell.