Think of the insert as a "loop" on a channel...stuff goes out to a processor and comes back in.
If it's a single jack, then like jimmys69 said...it's TRS, and you will be able to insert, but at the same time you will break the existing signal that's plugged into that channel.
If there are two jacks (rare on channel inserts), one for Send and the other for Receive (sorta' like you find on guitar amp FX loops)...then you could add a signal via the Receive, but it would be combined with the existing signal already on the channel.