When talking about filters in the context of electronics, it means a circuit that will block signals of a certain frequency and allow others to pass; some examples are signal filters to block signals of a certain frequency to be amplified, and source filters that remove an alternating component from a DC power line.
Most filters rely on the ability of some components, capacitors and inductors, to change their ability to conduct current at certain frequencies to attenuate a signal to the point where it's no longer a problem.
One thing with this "passive" approach is that you can't use multiple stages of filtering because the signal gets smaller smaller with every stage, so the use of "active" filters, those that incorporate an amplifying element, became a necessity.
In this series we have already used two of the most fundamental active filters, even though we didn't see them as such at the time, now you are going to revisit them with new eyes and new information on how exactly their filtering properties emerge.
