Time, UTC, Julian Date, TLE epoch - how are they related quantitatively?

I am trying to understand the relationship (quantitatively) between JulianDate, UTC, and the epoch of Two Line Elements. It's a deep subject, especially if you include relativity, and a long subject - there is a lot of history. But here what I need is just a bare-bones understanding of how these three things are related quantitatively, on earth, plus/minus a few years from now, to milli-second accuracy.

I roughly understand that UTC increase by 1 second every second, with the usual rules for hours, minutes, days, years, leap years and leap seconds.

I don't understand JulianDate. That links to a long article which states many facts, but after reading, I still don't understand what it is quantitatively. If I line up UTC agains Julian Day are they parallel (except for UTC's leap years/seconds)? Is there a mapping that I can understand or do I have to use a computer program or web site or some other black box to convert between the two?

Here are two examples of situations where I feel lost. I print additional information to make sure I understand some things - the questions are in bold:

Example 1 - time on my computer and in Skyfield:

from skyfield.api import now, JulianDate, loadimport timedef test():    a, b =  now(), time.gmtime()   # check the times    print a.tt_tuple()    print time.asctime(b)test()(2016.0, 1.0, 31.0, 11, 45, 28.927114605903625)Sun Jan 31 11:44:20 2016

Both of those are derived from my computer's system time (I think!) which is up-to-date. There is a difference of about 69 seconds. What is that?

Example 2TLE epoch:

from skyfield.api import now, JulianDate, loadimport mathdata = load('de421.bsp')earth = data['earth']TLE = """1 25544U 98067A   16031.25992506  .00006019  00000-0  97324-4 0  99942 25544  51.6430  25.8646 0006733  62.5910  66.7566 15.54344726983528"""ISS = earth.satellite(TLE)print ISS._sgp4_satellite.satnum, "  NORAD satellite number of ISS in the TLE"print ISS._sgp4_satellite.epoch,  "  The epoch of the TLE"frac = 0.25992506                 #  fraction of day in the epoch of the TLE"print round((((frac*24.-6.)*60.-14.)*60.-17.)*1E+06), "  the microseconds of the epoch"print ISS._sgp4_satellite.mo*(180./math.pi),    "  TLE Mean Anomaly - of ISS *at* epoch?"print JulianDate(utc=(2016, 1, 31, 6, 14, 17.525184)).tt,  "  What is this number??"

output:

25544   NORAD satellite number of ISS in the TLE2016-01-31 06:14:17.525184   The epoch of the TLE525184.0   the microseconds of the epoch66.7566   TLE Mean Anomaly - of ISS *at* epoch?2457418.76071   What is this number??

What is the exact meaning of the last number, and how can I convert the value of this epoch to UTC (if I wanted to)?