Sow for yourselves righteousness,
reap the fruit of unfailing love,
and break up your unplowed ground;
for it is time to seek the LORD,
until he comes
and showers righteousness on you.
We reap what we sow is an often repeated saying which has roots in passages such as Hosea 10. If we do good deeds, we’ll receive good things in return. We will get what we deserve, both good and bad.
It depends on one’s perspective whether the process of reaping and sowing is really true. Without a doubt there are bad people who seem to prosper and good people who suffer.
On a spiritual level, we will reap eternal benefits or suffer punishment based on what we have sown. Those who ignore the Lord now will be ignored when they cry out to Him on Judgment Day. This much is guaranteed.
As for what we receive on earth, reaping what we sow should be considered a proverb. It is good advice and generally true but not a promise of things to come. God does bless and curse people here on earth but not always. Why some people do not receive blessings and curses on earth is unanswerable as we do not know the mind of God.
There is a human element though as well. It is the Golden Rule in effect. If we treat people nicely, they are more likely to treat us nicely back. If we treat people poorly, we’re likely to be treated poorly in return.