A color space defines the range (gamut) of colors that can be represented in an image. Different color spaces cover different portions of visible light. sRGB is the standard for web and consumer devices — it covers about 35% of all visible colors. Adobe RGB covers about 50% of visible colors, capturing more saturated greens and cyans useful for print. Display P3 (used by Apple devices since 2016) covers about 40% of visible colors, encompassing most of sRGB plus wider color range.
Color spaces are embedded in image files as ICC profiles — metadata describing which color space the image uses. Without this metadata, display software assumes sRGB. An Adobe RGB image displayed without color management looks desaturated — the colors are "wrong" because the wide-gamut values are misinterpreted as narrow-gamut sRGB values.
For web use, always save images as sRGB — it's the universal standard that displays correctly on every device. For print, Adobe RGB captures more of the CMYK color gamut that offset printing can reproduce. Convert to sRGB for web: most image editors have "Convert to Profile" (not "Assign Profile") under color management settings. FileCurve's conversions output sRGB-compatible images suitable for web use.