The term "lack of" means:

- there is not enough of something

- there is an insufficient amount of something or an insufficient number of things.

I could not think well because of a lack of sleep (i.e., I was too tired)

I could not pay my bills becasue of a lack of funds in my bank account (i.e., not enough money to pay my bills)

I was not elected because of a lack of support from the voters.

People were thirsty due to a lack of drinking water.

The plants died due to a lack of rain.