I pay $1 to buy an apple and eat it. It is gone and tomorrow I will have no Apples unless I buy more. Instead I buy (invest in) an apple tree and now I have apples continuously.
The difference is the spending for a one time use vs production of new goods.
In the context of the thread, both investment and consumption are spending though. Whether one buys a shiny new gadget or a tech stock that pays dividends doesn't matter, because cost of doing either will be less tomorrow in a deflationary economy.
Consumption is concerned with your actual wants and needs. Investing is purely a financial measure with your excess cash. If you assess that it is financially savvier to not invest you will not invest. Investment is the concern of the large amount of excess money that wealthy people have that drive new projects.
But you’re still going to want and need things to consume. Much, much less sensitive to inflation.
The difference is the spending for a one time use vs production of new goods.