The significance of 'alt text' in web accessibility lies in its role in providing descriptions for visual content, such as images or videos, to users with disabilities. Alt text enables screen readers to convey the meaning and context of images to visually impaired individuals, thus making web content more inclusive. It also plays a crucial part in improving SEO by helping search engines understand the content of images. Proper use of alt text not only meets accessibility standards and enhances user experience but also broadens the reach of content, making it accessible to a diverse audience.