Basically if you took a loan from someone and he comes back repeatedly to ask you for it, he'd be doing taaqeedgi, or taaqeedein.
Taakeed is to assert something. To say with a force, pressure.
Frankly, some people are saying it's 'warning' which is not really the case. It may be close or related to the word depending on the usage, but can hardly be used as 'warning'.