To understand this, we must first acknowledge few premises of modern monetary theory:
1.The total amount consumed(spend) equates to the total price of products produced.
2.Bank will create money in the form of lending (eg banks only have 5 dollar in cash , but the society deposited totally 10 dollars in bank , this is due to most people wont hold all their savings in cash at home, but rather deposit them in the bank, and bank could lend out those cash again.)
These form the foundation of QTM( Quantitative theory of Money)
OR
(Money that is cash)<M>x (total amount of times they are circulated in society)<V>
=price level of product times<P>the total quantity of product<Y>(how much the society spends in total)
*It is easier to understand this as the total value of production equals to how much money is in society times the time they switched hands, as people will buy them all.
AKA: MV=PY
so why would printing money(increase in cash) causes inflation?
In very simple words, V is affected by spending behavior or technological production
while Y is affected by real factors of production ( lomo of those who open post without picture), and are very unlikely to move in a short time.
Thus , we can assume that these two factors are constant, and as such:
M=P or printing money will cause the price level of product to increase.