Why did the french and the begians invade the ruhr in 1923?

Because the Treaty of Versailles stated that Germany must pay a large amount of money (war reparations) towards the Allies after the WW1.

For many reasons, Germany could not afford these payments, their economy, industry, and basically the whole country would just collapse if they were to continue to pay.

The French and Belgians, being the main 'victims' in the war, were outraged, did not listen to the excuses, and invaded the Ruhr Valley in Germany.

The Ruhr was Germany's main industrial area and thus - their main supply of wealth.

This basically meant that the French and Belgians could get the money that Germany could not afford to pay them.