Celtic 0 (-150)
Rangers 0 (+115)

When the spread in soccer is 0, are you basically betting for a tie or your team wins?

It means :
The game starts at 0-0.
When the bettor gives away -1.5 ball ( Celtic ), he is taking Celtic at minus 1.5 goals before the game starts. Meaning, Celtic are starting the game with a 1.5 goal deficit. That means he can only win if Celtic win the game by at least 2 clear goal margin.
Example : 2-0, 3-0, 3-1, 4-0,4-1,4-2, 5-0, 5-1... etc...

When the bettor takes +1.5 ( Rangers ), he is taking Rangers at +1.5 goal before the game starts. Meaning, Rangers are starting the game leading by 1.5 goals.
As long as Rangers do not loose by a 2 goal margin, the bettor wins.
Example : If Rangers win, draw, or if Rangers loose by 1 goal margin.
Loose 1-0. 2-1, 3-2, 4-3... etc.

The reason the handicap is 1.5 is because, there will always be a winner and a looser. If the bettor gives -2 goals, and Celtic win 2-0, then no one wins. 1-5 handicap assures someone wins or looses.