Bootstrapping is the process of starting and funding business on your own without taking funds from any external investors.
A bootstrapped company goes through the following stages:
1. Personal fund: Starting business by investing your personal funds to carry out the business operations.
2. Customer fund: At this stage company covers its expenses from the profits that it is earning.
3. External fund: It is a stage where the company needs to look for external funds to grow operations.
You can read about the advantages and disadvantages of bootstrapping in the article linked here: https://www.feedough.com/startup-bootstrapping/