The Morning Line is the name of the track's starting point on the morning of race day, predicting how the public's betting will affect the payoff odds. It's free at the track. Race books have their own version of the morning line, which is part of the race sheet produced at the book for any particular track and race day. It is like the race program at the track, in that it lists the horses and races, but it also gives the predicted odds.
It is an easy mistake to treat the morning line as if it were the race book's prediction on how the race will be run. It is not that at all. It is a prediction about how the pari-mutuel betting pool will result. It is for this very reason that the morning line is just a starting point for one line of thinking in handicapping. If it turns out that the odds close to post time are vastly different from the morning line, it is worth asking oneself why. Is there something the public knows or thinks it knows, which the line-maker (the "house" or the track) didn't know? Or vice-versa? If so, which is closer to being correct? Is there an overlay here?
Absent some specific information that leads you to a different conclusion, the morning line can be taken as one experienced handicapper's prediction about how the public will bet, assuming a fairly smart public. So it is persuasive, even though preliminary.
Perhaps "starting point" is misleading, as the morning line is normally only available on or very close to race day, and the handicapping process should begin sooner than that. People who wait until they are in the book before handicapping are probably handicapping themselves.