What do people actually mean by 'overtrading'?
As stated above, it's:
A: Placing too many orders (though generally if there is a good reason, it's ok if managed well.)
B: Placing orders where the lot size is too large (this is the number one killer of an account.)
Both can be alleviated by keeping your margin level above a threshold. The higher your leverage, the higher your threshold should be, generally the consensus is that an above 1000% margin level should be adequate up to 1:1000 leverage, I somewhat disagree and think above 1000% up to 1:100 leverage, and 1000% for every additional 100 (eg. 1:200 - 2000%) will provide a good buffer for any major movement. The correct way to increase your margin level is to deposit additional funds and take losses off the table, a beginner mistake is to attempt a correction by opening additional orders (covering losses), thereby 'overtrading'.
Covering losses - opening additional orders in the hope that the profit from them will cancel out or exceed the orders running at a loss.
For every loss there should be at least an equal and opposite profit.