Resizing text generally means increasing the size of the text on a website to make it easier to read. When text is resized up to 200%, there should be no loss of content or layout functionality. In other words, content should not "break" for site visitors.
People with low vision sometimes need to increase the size of text so they can read it.
There has been a bit of controversy about whether text resizing means resizing (zooming in on) the whole page or just resizing the text. For the purposes here, we interpret the requirement to mean resizing text only. In addition, while the relevant reference, Understanding Success Criterion 1.4.4 specifies 200%, you may encounter resources that suggest that 150% may be adequate. If you choose the lower percentage as acceptable for text resizing, take care to document this choice and why you made it.
The best way to check that text can be resized without loss of content or diminished layout functionality is to display a website in a browser and increase the size of the text only to 200%. You should be able to read everything on the page.