What fluid typography means
Classic responsive type picks a few fixed sizes and swaps them with media queries: 16px on phones, 18px on tablets, 20px on desktop. Fluid typography replaces those steps with a continuous function of the viewport width. With CSS clamp() the whole rule fits on one line, and the browser calculates the right size for every width between your two endpoints.
The formula behind clamp()
A fluid size is a straight line through two points: (min viewport, min size) and (max viewport, max size). The slope is (max size − min size) ÷ (max viewport − min viewport). Multiply it by 100 to get the vw coefficient. The intercept is min size − slope × min viewport. The result is clamp(min size, intercept + slope·100vw, max size). The generator above does exactly this and rounds to the number of decimals you choose.
Choosing good sizes
Body text rarely needs to change much: 16 to 18 or 16 to 20 pixels covers most reading layouts. Headings benefit more from fluid sizing because a 48px headline that looks great on a laptop takes up half a phone screen. Start with your largest heading and work down, keeping the ratio between min and max smaller for smaller text.
Pick the viewport range from your layout, not from device lists. The minimum is usually 320 or 360 px. The maximum is the width where your content container reaches its max-width; past that point the line length no longer grows, so the font size should not either.
Accessibility checklist
Keep min, max and the intercept in rem so user font preferences still apply. Check WCAG 1.4.4 by zooming to 200%: when the maximum is more than about 2.5 times the minimum, zoom may not enlarge text enough, and the tool warns you. Keep line height relative (for example 1.5) so it scales with the font size, and test your longest words on the narrowest screen.