Odds reflect how likely an outcome is. During a live event the situation changes all the time, so the odds are constantly updated to match it.
Odds can change after:
a goal, point or any other score change
a red card, penalty or injury
time passing, because the closer the game is to the end, the less time there is for the result to change
betting activity on the market
For example, Team A is leading 1-0. In the 20th minute, the odds for Team A to win might be 1.60. In the 85th minute, with the score still 1-0, the same outcome might be at 1.10, because Team B has very little time left to equalise.
Lower odds mean an outcome is more likely, and higher odds mean it is less likely. Once a bet is accepted, it keeps the odds it was placed at. Later changes do not affect it.