Bad seeing on a clear night? I want to say first off, I am a complete beginner, starting out my interest in astronomy (so go easy!). 

I have been able to see some nice sights on some nights, however, I had a strange experience the other night when viewing Saturn.

I had been viewing it the night before, using my highest magnification eyepiece and got some really nice views. It can be noted my telescope has a focal length of 1000mm and aperture of 100mm and I was using a 5mm eyepiece, which was really pushing the limits of the useable magnification, as far as I am aware.

So anyway, the next night had clear skies again, if anything maybe clearer. I was excited to see Saturn again, but this time once I had found it, I put in my 5mm eyepiece and just couldn't get it into the same focus I had the previous night. I googled it and somewhere said that although the skies can be good, the seeing can be poor. 

What is meant by this statement and how am I able to tell if there is going to be a night with poor seeing in advance so I know which eyepieces I can use for that particular session? The app I use to check the skies only tells me if it is going to be clear so I don;t know what to be looking out for.
Seeing refers to turbulence in the atmosphere that distorts wavefronts passing through and results in the blurring of distant objects.

Unfortunately, there is no foolproof way of predicting what the seeing will be from night to night, though building up a database of local seeing and meteorological measurements might allow you to find some crude correlations using principal component analysis.