convolution
When a signal go through a linear system, its frequency components are affected by the linear system, the frequencies results is a weight composition of the system frequency response. What happens is a multiplication in frequency not in time. You will learn that multiplication in frequency domaing is equivalent of convolution in time domain. Multipling signals in time domain is associated with displacement in frequency, for instance modulation process, wich for its turn is equivalent of convolution in frequency.
Go ahead in your studying and you will son realise the diferrences.