An early Buddhist teaching says:
What we are today comes from our thoughts of yesterday, and our present thoughts build our life of tomorrow. Our life is the creation of our mind.
Karma means intended action, and is a dynamic concept. It is not fate or predestination, but a consequence of what has gone before. In other words, you are now in circumstances because of your thoughts and decisions, and this is an on-going process. That is, new actions create new Karma.
Intention is a key part of Karma. If you come home and accidentally trip over your daughter and hurt her, this is not intended and has no effect.
However, if you come home and kick your daughter intentionally but for no reason, then negative Karma is generated. All your combined intended actions add up to what you are now.
The Buddha saw this as an explanation of the different circumstances that all living beings find themselves in.
Karma is linked with dependent origination, where it is the consequence of the law of cause and effect. In the Bible, it says that we reap what we sow, and karma has the same impact.
We often associate Karma with fate and it has this idea of a future which is predetermined. In Buddhism, Karma is the reaping of past actions, but also offers the possibility of determining our own future with our actions and thoughts today.
This means that no future event is locked in, and what happens today and tomorrow will create the personal and global World of the future.
Never forget yesterday, but always live for today, because you never know what tomorrow can bring, or what it can take away.