The short answer is that efficiency is the measure of how long the machine is actually crafting vs. doing nothing at all.
There can be several reasons for doing nothing -- the most common being a lack of sufficient input items, but another cause is having too many output items (when the output is full, the machine doesn't run).
If your efficiency is low due to insufficient input items, then you need to build more machines to produce more items, increase the clock on upstream machines, or increase the conveyor speed (or decrease the distance) so that the existing items arrive faster.
If your efficiency is low due to too many output items, then you either need to reduce the clock on the machine or add splitters to the input resources to send them elsewhere. Or look further downstream to find out why the output of this machine is backstuffing -- maybe there's something else to speed up down there, or you need to increase the output belt speed.
But low efficiency is not in itself a bad thing -- even a perfect factory would eventually become almost 0% efficient once all the storage fills up -- and that's fine.
It's fun to try to optimise your efficiency, but don't worry about it too much, especially early game before you have access to faster belts.