Second Law tells us that it is not possible to have a process in which the ONLY result will be to transfer energy as heat from a hot to a cold body. This means that some of the heat is...basically wasted and will not be available!
Let us say that we extract 100 of energy from a hot reservoir and we use it to do work we will not get 100 of work because we will need to waste a bit in the environment (say, 20) as the cold reservoir.
The efficiency will then be less than 100%.
This sounds a bit weird and difficult but there is an interesting example that can help us:
Suppose you can change entirely heat into work; so you extract 100 and do 100 of work.
Now this work could be used to extract 150 of heat from the environment and send it back to our hot reservoir. The ending result would be to transfer 50 of heat from a cold reservoir to a hot one WITHOUT any intervention, spontaneously....which is impossible (or better, improbable, because up to now never happened according to Boltzmann!).
Hope I didn't confuse you more!