“Truly I tell you, this poor widow has offered more than all of those who made offerings to the coffers. Because they may have offered a lot – but she offered all that she had, and all she had to live on.” (Mark 12:43-44)

Here is the situation:
Then Jesus sat down in front of the temple offering coffers and saw how the crowd offered coins into the temple coffers. Some rich people offered large amounts. A poor widow approached and placed two small coins in – a very small amount of money. (Mark 12:41-42)
Jesus is referring to devotion here. Money is not the issue here. Otherwise, Jesus would have praised those who put in more money.

Rather, Jesus is speaking of the widow's devotion to God. She had very little money. Yet she put in money that she needed to feed herself. Instead, she decided it was more important to give that money to the temple.

This sort of devotion is beyond most of us. We have little access to this devotional position because our hearts are hard. We are focused on ourselves. We are focused on our body's survival. We are focused on what others think of us. We are focused on our careers, or our family, or our house, or our car. Or all of the above combined. These are the things that concern us the most.

This woman's concerns were single-pointed upon pleasing God. This is why Jesus pointed her out to his students. Jesus wanted to show his students what it was like to be devoted to the Supreme Being.

This is also the central doctrine of Jesus. Loving devotion to God was his central message and his purpose for descending here. This is illustrated by most important instruction:
“The most important of all the instructions is, ‘Hear O Israel – the LORD our God is our only Lord – and you shall love the LORD your God with all your heart, with all your soul and with all your mind and with all your strength’ – this is the most important instruction." (Mark 12:29-30)

(The New Testament verses in this article are quoted from the Gospels of Jesus).